diff --git a/packages/core/index.js b/packages/core/index.js
index 0985137e7..5b06d917f 100644
--- a/packages/core/index.js
+++ b/packages/core/index.js
@@ -1,8 +1,3 @@
-/**
- * Info for TypeScript users:
- *   For now please import types from lit-element and lit-html directly.
- */
-
 // lit-element
 export {
   css,
diff --git a/packages/core/package.json b/packages/core/package.json
index 8f19358eb..c0a367df0 100644
--- a/packages/core/package.json
+++ b/packages/core/package.json
@@ -13,11 +13,14 @@
   "main": "index.js",
   "module": "index.js",
   "files": [
+    "*.d.ts",
     "*.js",
     "docs",
     "src",
+    "test",
     "test-helpers",
-    "translations"
+    "translations",
+    "types"
   ],
   "scripts": {
     "prepublishOnly": "../../scripts/npm-prepublish.js",
@@ -26,7 +29,7 @@
   },
   "sideEffects": false,
   "dependencies": {
-    "@open-wc/dedupe-mixin": "^1.2.1",
+    "@open-wc/dedupe-mixin": "^1.2.18",
     "@open-wc/scoped-elements": "^1.0.3",
     "lit-element": "^2.2.1",
     "lit-html": "^1.0.0"
diff --git a/packages/core/src/DelegateMixin.js b/packages/core/src/DelegateMixin.js
index 04f5ee9a1..3c4dba16c 100644
--- a/packages/core/src/DelegateMixin.js
+++ b/packages/core/src/DelegateMixin.js
@@ -1,203 +1,177 @@
 /* eslint-disable class-methods-use-this */
-
 import { dedupeMixin } from '@open-wc/dedupe-mixin';
 
 /**
- * # DelegateMixin
- * Forwards defined events, methods, properties and attributes to the defined target.
- *
- * @example
- * get delegations() {
- *   return {
- *     ...super.delegations,
- *     target: () => this.shadowRoot.getElementById('button1'),
- *     events: ['click'],
- *     methods: ['click'],
- *     properties: ['disabled'],
- *     attributes: ['disabled'],
- *   };
- * }
- *
- * render() {
- *   return html`
- *     <button id="button1">with delegation</button>
- *   `;
- * }
- *
- * @type {function()}
- * @polymerMixin
- * @mixinFunction
+ * @typedef {import('../types/DelegateMixinTypes').DelegateMixin} DelegateMixin
  */
-export const DelegateMixin = dedupeMixin(
-  superclass =>
-    // eslint-disable-next-line
-    class DelegateMixin extends superclass {
-      constructor() {
-        super();
-        this.__eventsQueue = [];
-        this.__propertiesQueue = {};
-        this.__setupPropertyDelegation();
-      }
 
-      /**
-       * @returns {{target: null, events: Array, methods: Array, properties: Array, attributes: Array}}
-       */
-      get delegations() {
-        return {
-          target: null,
-          events: [],
-          methods: [],
-          properties: [],
-          attributes: [],
-        };
-      }
+/**
+ * @typedef DelegateEvent
+ * @property {string} type - Type of event
+ * @property {Array<?>} args - Event arguments
+ */
 
-      connectedCallback() {
-        if (super.connectedCallback) {
-          super.connectedCallback();
-        }
-        this._connectDelegateMixin();
-      }
+/** @type {DelegateMixin} */
+const DelegateMixinImplementation = superclass =>
+  // eslint-disable-next-line
+  class DelegateMixin extends superclass {
+    constructor() {
+      super();
 
-      updated(...args) {
-        super.updated(...args);
-        this._connectDelegateMixin();
-      }
+      /** @type {DelegateEvent[]} */
+      this.__eventsQueue = [];
 
-      /**
-       * @param {string} type
-       * @param {Object} args
-       */
-      addEventListener(type, ...args) {
-        const delegatedEvents = this.delegations.events;
-        if (delegatedEvents.indexOf(type) > -1) {
-          if (this.delegationTarget) {
-            this.delegationTarget.addEventListener(type, ...args);
-          } else {
-            this.__eventsQueue.push({ type, args });
-          }
+      /** @type {Object.<string,?>} */
+      this.__propertiesQueue = {};
+      this.__setupPropertyDelegation();
+    }
+
+    /**
+     * @returns {{target: Function, events: string[], methods: string[], properties: string[], attributes: string[]}}
+     */
+    get delegations() {
+      return {
+        target: () => {},
+        events: [],
+        methods: [],
+        properties: [],
+        attributes: [],
+      };
+    }
+
+    connectedCallback() {
+      if (super.connectedCallback) {
+        super.connectedCallback();
+      }
+      this._connectDelegateMixin();
+    }
+
+    /** @param {import('lit-element').PropertyValues } changedProperties */
+    updated(changedProperties) {
+      super.updated(changedProperties);
+      this._connectDelegateMixin();
+    }
+
+    /**
+     * @param {string} type
+     * @param {...Object} args
+     */
+    addEventListener(type, ...args) {
+      const delegatedEvents = this.delegations.events;
+      if (delegatedEvents.indexOf(type) > -1) {
+        if (this.delegationTarget) {
+          this.delegationTarget.addEventListener(type, ...args);
         } else {
-          super.addEventListener(type, ...args);
+          this.__eventsQueue.push({ type, args });
         }
+      } else {
+        super.addEventListener(type, ...args);
       }
+    }
 
-      /**
-       * @param {string} name
-       * @param {string} value
-       */
-      setAttribute(name, value) {
-        const attributeNames = this.delegations.attributes;
-        if (attributeNames.indexOf(name) > -1) {
-          if (this.delegationTarget) {
-            this.delegationTarget.setAttribute(name, value);
-          }
-          super.removeAttribute(name);
-        } else {
-          super.setAttribute(name, value);
-        }
-      }
-
-      /**
-       * @param {string} name
-       */
-      removeAttribute(name) {
-        const attributeNames = this.delegations.attributes;
-        if (attributeNames.indexOf(name) > -1) {
-          if (this.delegationTarget) {
-            this.delegationTarget.removeAttribute(name);
-          }
+    /**
+     * @param {string} name
+     * @param {string} value
+     */
+    setAttribute(name, value) {
+      const attributeNames = this.delegations.attributes;
+      if (attributeNames.indexOf(name) > -1) {
+        if (this.delegationTarget) {
+          this.delegationTarget.setAttribute(name, value);
         }
         super.removeAttribute(name);
+      } else {
+        super.setAttribute(name, value);
       }
+    }
 
-      /**
-       * @protected
-       */
-      _connectDelegateMixin() {
-        if (this.__connectedDelegateMixin) return;
-
-        if (!this.delegationTarget) {
-          this.delegationTarget = this.delegations.target();
-        }
-
+    /**
+     * @param {string} name
+     */
+    removeAttribute(name) {
+      const attributeNames = this.delegations.attributes;
+      if (attributeNames.indexOf(name) > -1) {
         if (this.delegationTarget) {
-          this.__emptyEventListenerQueue();
-          this.__emptyPropertiesQueue();
-          this.__initialAttributeDelegation();
-
-          this.__connectedDelegateMixin = true;
+          this.delegationTarget.removeAttribute(name);
         }
       }
+      super.removeAttribute(name);
+    }
 
-      /**
-       * @private
-       */
-      __setupPropertyDelegation() {
-        const propertyNames = this.delegations.properties.concat(this.delegations.methods);
-        propertyNames.forEach(propertyName => {
-          Object.defineProperty(this, propertyName, {
-            get() {
-              const target = this.delegationTarget;
-              if (target) {
-                if (typeof target[propertyName] === 'function') {
-                  return target[propertyName].bind(target);
-                }
-                return target[propertyName];
+    _connectDelegateMixin() {
+      if (this.__connectedDelegateMixin) return;
+
+      if (!this.delegationTarget) {
+        this.delegationTarget = this.delegations.target();
+      }
+
+      if (this.delegationTarget) {
+        this.__emptyEventListenerQueue();
+        this.__emptyPropertiesQueue();
+        this.__initialAttributeDelegation();
+
+        this.__connectedDelegateMixin = true;
+      }
+    }
+
+    __setupPropertyDelegation() {
+      const propertyNames = this.delegations.properties.concat(this.delegations.methods);
+      propertyNames.forEach(propertyName => {
+        Object.defineProperty(this, propertyName, {
+          get() {
+            const target = this.delegationTarget;
+            if (target) {
+              if (typeof target[propertyName] === 'function') {
+                return target[propertyName].bind(target);
               }
-              if (this.__propertiesQueue[propertyName]) {
-                return this.__propertiesQueue[propertyName];
+              return target[propertyName];
+            }
+            if (this.__propertiesQueue[propertyName]) {
+              return this.__propertiesQueue[propertyName];
+            }
+            // This is the moment the attribute is not delegated (and thus removed) yet.
+            // and the property is not set, but the attribute is (it serves as a fallback for
+            // __propertiesQueue).
+            return this.getAttribute(propertyName);
+          },
+          set(newValue) {
+            if (this.delegationTarget) {
+              const oldValue = this.delegationTarget[propertyName];
+              this.delegationTarget[propertyName] = newValue;
+              // connect with observer system if available
+              if (typeof this.triggerObserversFor === 'function') {
+                this.triggerObserversFor(propertyName, newValue, oldValue);
               }
-              // This is the moment the attribute is not delegated (and thus removed) yet.
-              // and the property is not set, but the attribute is (it serves as a fallback for
-              // __propertiesQueue).
-              return this.getAttribute(propertyName);
-            },
-            set(newValue) {
-              if (this.delegationTarget) {
-                const oldValue = this.delegationTarget[propertyName];
-                this.delegationTarget[propertyName] = newValue;
-                // connect with observer system if available
-                if (typeof this.triggerObserversFor === 'function') {
-                  this.triggerObserversFor(propertyName, newValue, oldValue);
-                }
-              } else {
-                this.__propertiesQueue[propertyName] = newValue;
-              }
-            },
-          });
+            } else {
+              this.__propertiesQueue[propertyName] = newValue;
+            }
+          },
         });
-      }
+      });
+    }
 
-      /**
-       * @private
-       */
-      __initialAttributeDelegation() {
-        const attributeNames = this.delegations.attributes;
-        attributeNames.forEach(attributeName => {
-          const attributeValue = this.getAttribute(attributeName);
-          if (typeof attributeValue === 'string') {
-            this.delegationTarget.setAttribute(attributeName, attributeValue);
-            super.removeAttribute(attributeName);
-          }
-        });
-      }
+    __initialAttributeDelegation() {
+      const attributeNames = this.delegations.attributes;
+      attributeNames.forEach(attributeName => {
+        const attributeValue = this.getAttribute(attributeName);
+        if (typeof attributeValue === 'string') {
+          this.delegationTarget.setAttribute(attributeName, attributeValue);
+          super.removeAttribute(attributeName);
+        }
+      });
+    }
 
-      /**
-       * @private
-       */
-      __emptyEventListenerQueue() {
-        this.__eventsQueue.forEach(ev => {
-          this.delegationTarget.addEventListener(ev.type, ...ev.args);
-        });
-      }
+    __emptyEventListenerQueue() {
+      this.__eventsQueue.forEach(ev => {
+        this.delegationTarget.addEventListener(ev.type, ...ev.args);
+      });
+    }
 
-      /**
-       * @private
-       */
-      __emptyPropertiesQueue() {
-        Object.keys(this.__propertiesQueue).forEach(propName => {
-          this.delegationTarget[propName] = this.__propertiesQueue[propName];
-        });
-      }
-    },
-);
+    __emptyPropertiesQueue() {
+      Object.keys(this.__propertiesQueue).forEach(propName => {
+        this.delegationTarget[propName] = this.__propertiesQueue[propName];
+      });
+    }
+  };
+
+export const DelegateMixin = dedupeMixin(DelegateMixinImplementation);
diff --git a/packages/core/src/DisabledMixin.js b/packages/core/src/DisabledMixin.js
index 3119670f6..1dea190e8 100644
--- a/packages/core/src/DisabledMixin.js
+++ b/packages/core/src/DisabledMixin.js
@@ -1,64 +1,67 @@
 import { dedupeMixin } from '@open-wc/dedupe-mixin';
 
 /**
- * #DisabledMixin
- *
- * @polymerMixin
- * @mixinFunction
+ * @typedef {import('../types/DisabledMixinTypes').DisabledMixin} DisabledMixin
  */
-export const DisabledMixin = dedupeMixin(
-  superclass =>
-    // eslint-disable-next-line no-shadow
-    class DisabledMixin extends superclass {
-      static get properties() {
-        return {
-          disabled: {
-            type: Boolean,
-            reflect: true,
-          },
-        };
-      }
 
-      constructor() {
-        super();
+/** @type {DisabledMixin} */
+const DisabledMixinImplementation = superclass =>
+  // eslint-disable-next-line no-shadow
+  class DisabledMixinHost extends superclass {
+    static get properties() {
+      return {
+        disabled: {
+          type: Boolean,
+          reflect: true,
+        },
+      };
+    }
+
+    constructor() {
+      super();
+      this.__requestedToBeDisabled = false;
+      this.__isUserSettingDisabled = true;
+      this.__restoreDisabledTo = false;
+      this.disabled = false;
+    }
+
+    makeRequestToBeDisabled() {
+      if (this.__requestedToBeDisabled === false) {
+        this.__requestedToBeDisabled = true;
+        this.__restoreDisabledTo = this.disabled;
+        this.__internalSetDisabled(true);
+      }
+    }
+
+    retractRequestToBeDisabled() {
+      if (this.__requestedToBeDisabled === true) {
         this.__requestedToBeDisabled = false;
-        this.__isUserSettingDisabled = true;
-
-        this.__restoreDisabledTo = false;
-        this.disabled = false;
+        this.__internalSetDisabled(this.__restoreDisabledTo);
       }
+    }
 
-      makeRequestToBeDisabled() {
-        if (this.__requestedToBeDisabled === false) {
-          this.__requestedToBeDisabled = true;
+    /** @param {boolean} value */
+    __internalSetDisabled(value) {
+      this.__isUserSettingDisabled = false;
+      this.disabled = value;
+      this.__isUserSettingDisabled = true;
+    }
+
+    /**
+     * @param {PropertyKey} name
+     * @param {?} oldValue
+     */
+    _requestUpdate(name, oldValue) {
+      super._requestUpdate(name, oldValue);
+      if (name === 'disabled') {
+        if (this.__isUserSettingDisabled) {
           this.__restoreDisabledTo = this.disabled;
+        }
+        if (this.disabled === false && this.__requestedToBeDisabled === true) {
           this.__internalSetDisabled(true);
         }
       }
+    }
+  };
 
-      retractRequestToBeDisabled() {
-        if (this.__requestedToBeDisabled === true) {
-          this.__requestedToBeDisabled = false;
-          this.__internalSetDisabled(this.__restoreDisabledTo);
-        }
-      }
-
-      __internalSetDisabled(value) {
-        this.__isUserSettingDisabled = false;
-        this.disabled = value;
-        this.__isUserSettingDisabled = true;
-      }
-
-      _requestUpdate(name, oldValue) {
-        super._requestUpdate(name, oldValue);
-        if (name === 'disabled') {
-          if (this.__isUserSettingDisabled) {
-            this.__restoreDisabledTo = this.disabled;
-          }
-          if (this.disabled === false && this.__requestedToBeDisabled === true) {
-            this.__internalSetDisabled(true);
-          }
-        }
-      }
-    },
-);
+export const DisabledMixin = dedupeMixin(DisabledMixinImplementation);
diff --git a/packages/core/src/DisabledWithTabIndexMixin.js b/packages/core/src/DisabledWithTabIndexMixin.js
index e0570bacf..0ecea056e 100644
--- a/packages/core/src/DisabledWithTabIndexMixin.js
+++ b/packages/core/src/DisabledWithTabIndexMixin.js
@@ -2,85 +2,91 @@ import { dedupeMixin } from '@open-wc/dedupe-mixin';
 import { DisabledMixin } from './DisabledMixin.js';
 
 /**
- * #DisabledWithTabIndexMixin
- *
- * @polymerMixin
- * @mixinFunction
+ * @typedef {import('../types/DisabledWithTabIndexMixinTypes').DisabledWithTabIndexMixin} DisabledWithTabIndexMixin
  */
-export const DisabledWithTabIndexMixin = dedupeMixin(
-  superclass =>
-    // eslint-disable-next-line no-shadow
-    class DisabledWithTabIndexMixin extends DisabledMixin(superclass) {
-      static get properties() {
-        return {
-          // we use a property here as if we use the native tabIndex we can not set a default value
-          // in the constructor as it synchronously sets the attribute which is not allowed in the
-          // constructor phase
-          tabIndex: {
-            type: Number,
-            reflect: true,
-            attribute: 'tabindex',
-          },
-        };
+
+/** @type {DisabledWithTabIndexMixin} */
+const DisabledWithTabIndexMixinImplementation = superclass =>
+  // eslint-disable-next-line no-shadow
+  class DisabledWithTabIndexMixinHost extends DisabledMixin(superclass) {
+    static get properties() {
+      return {
+        // we use a property here as if we use the native tabIndex we can not set a default value
+        // in the constructor as it synchronously sets the attribute which is not allowed in the
+        // constructor phase
+        tabIndex: {
+          type: Number,
+          reflect: true,
+          attribute: 'tabindex',
+        },
+      };
+    }
+
+    constructor() {
+      super();
+      this.__isUserSettingTabIndex = true;
+      this.__restoreTabIndexTo = 0;
+      this.__internalSetTabIndex(0);
+    }
+
+    makeRequestToBeDisabled() {
+      super.makeRequestToBeDisabled();
+      if (this.__requestedToBeDisabled === false && this.tabIndex != null) {
+        this.__restoreTabIndexTo = this.tabIndex;
       }
+    }
 
-      constructor() {
-        super();
-        this.__isUserSettingTabIndex = true;
-
-        this.__restoreTabIndexTo = 0;
-        this.__internalSetTabIndex(0);
+    retractRequestToBeDisabled() {
+      super.retractRequestToBeDisabled();
+      if (this.__requestedToBeDisabled === true) {
+        this.__internalSetTabIndex(this.__restoreTabIndexTo);
       }
+    }
 
-      makeRequestToBeDisabled() {
-        super.makeRequestToBeDisabled();
-        if (this.__requestedToBeDisabled === false) {
-          this.__restoreTabIndexTo = this.tabIndex;
-        }
-      }
+    /**
+     * @param {number} value
+     */
+    __internalSetTabIndex(value) {
+      this.__isUserSettingTabIndex = false;
+      this.tabIndex = value;
+      this.__isUserSettingTabIndex = true;
+    }
 
-      retractRequestToBeDisabled() {
-        super.retractRequestToBeDisabled();
-        if (this.__requestedToBeDisabled === true) {
+    /**
+     * @param {PropertyKey} name
+     * @param {?} oldValue
+     */
+    _requestUpdate(name, oldValue) {
+      super._requestUpdate(name, oldValue);
+
+      if (name === 'disabled') {
+        if (this.disabled) {
+          this.__internalSetTabIndex(-1);
+        } else {
           this.__internalSetTabIndex(this.__restoreTabIndexTo);
         }
       }
 
-      __internalSetTabIndex(value) {
-        this.__isUserSettingTabIndex = false;
-        this.tabIndex = value;
-        this.__isUserSettingTabIndex = true;
-      }
-
-      _requestUpdate(name, oldValue) {
-        super._requestUpdate(name, oldValue);
-
-        if (name === 'disabled') {
-          if (this.disabled) {
-            this.__internalSetTabIndex(-1);
-          } else {
-            this.__internalSetTabIndex(this.__restoreTabIndexTo);
-          }
+      if (name === 'tabIndex') {
+        if (this.__isUserSettingTabIndex && this.tabIndex != null) {
+          this.__restoreTabIndexTo = this.tabIndex;
         }
 
-        if (name === 'tabIndex') {
-          if (this.__isUserSettingTabIndex) {
-            this.__restoreTabIndexTo = this.tabIndex;
-          }
-
-          if (this.tabIndex !== -1 && this.__requestedToBeDisabled === true) {
-            this.__internalSetTabIndex(-1);
-          }
-        }
-      }
-
-      firstUpdated(changedProperties) {
-        super.firstUpdated(changedProperties);
-        // for ShadyDom the timing is a little different and we need to make sure
-        // the tabindex gets correctly updated here
-        if (this.disabled) {
+        if (this.tabIndex !== -1 && this.__requestedToBeDisabled === true) {
           this.__internalSetTabIndex(-1);
         }
       }
-    },
-);
+    }
+
+    /** @param {import('lit-element').PropertyValues } changedProperties */
+    firstUpdated(changedProperties) {
+      super.firstUpdated(changedProperties);
+      // for ShadyDom the timing is a little different and we need to make sure
+      // the tabindex gets correctly updated here
+      if (this.disabled) {
+        this.__internalSetTabIndex(-1);
+      }
+    }
+  };
+
+export const DisabledWithTabIndexMixin = dedupeMixin(DisabledWithTabIndexMixinImplementation);
diff --git a/packages/core/src/LionSingleton.js b/packages/core/src/LionSingleton.js
index 5353ff8c5..de864ab3f 100644
--- a/packages/core/src/LionSingleton.js
+++ b/packages/core/src/LionSingleton.js
@@ -6,10 +6,11 @@
  */
 export class LionSingleton {
   /**
-   * @param {function()} mixin
+   * @param {function} mixin
    */
   static addInstanceMixin(mixin) {
     if (!this.__instanceMixins) {
+      /** @type {function[]} */
       this.__instanceMixins = [];
     }
     this.__instanceMixins.push(mixin);
@@ -26,6 +27,8 @@ export class LionSingleton {
         Klass = mixin(Klass);
       });
     }
+    // Ignoring, because it's up to the extension layer to accept arguments in its constructor
+    // @ts-ignore-next-line
     return new Klass(...args);
   }
 
diff --git a/packages/core/src/SlotMixin.js b/packages/core/src/SlotMixin.js
index 5d1e81f76..7ccc31b34 100644
--- a/packages/core/src/SlotMixin.js
+++ b/packages/core/src/SlotMixin.js
@@ -1,78 +1,59 @@
 /* eslint-disable class-methods-use-this */
-
 import { dedupeMixin } from '@open-wc/dedupe-mixin';
+
 /**
- * # SlotMixin
- * `SlotMixin`, when attached to the DOM it creates content for defined slots in the Light DOM.
- * The content element is created using a factory function and is assigned a slot name from the key.
- * Existing slot content is not overridden.
- *
- * The purpose is to have the default content in the Light DOM rather than hidden in Shadow DOM
- * like default slot content works natively.
- *
- * @example
- * get slots() {
- *   return {
- *     ...super.slots,
- *     // appends <div slot="foo"></div> to the Light DOM of this element
- *     foo: () => document.createElement('div'),
- *   };
- * }
- *
- * @type {function()}
- * @polymerMixin
- * @mixinFunction
+ * @typedef {import('../types/SlotMixinTypes').SlotMixin} SlotMixin
+ * @typedef {import('../types/SlotMixinTypes').SlotsMap} SlotsMap
  */
-export const SlotMixin = dedupeMixin(
-  superclass =>
-    // eslint-disable-next-line no-unused-vars, no-shadow
-    class SlotMixin extends superclass {
-      /**
-       * @returns {{}}
-       */
-      get slots() {
-        return {};
-      }
 
-      constructor() {
-        super();
-        this.__privateSlots = new Set(null);
-      }
+/** @type {SlotMixin} */
+const SlotMixinImplementation = superclass =>
+  // eslint-disable-next-line no-unused-vars, no-shadow
+  class SlotMixinHost extends superclass {
+    /**
+     * @return {SlotsMap}
+     */
+    get slots() {
+      return {};
+    }
 
-      connectedCallback() {
-        if (super.connectedCallback) {
-          super.connectedCallback();
-        }
-        this._connectSlotMixin();
-      }
+    constructor() {
+      super();
+      this.__privateSlots = new Set(null);
+    }
 
-      /**
-       * @protected
-       */
-      _connectSlotMixin() {
-        if (!this.__isConnectedSlotMixin) {
-          Object.keys(this.slots).forEach(slotName => {
-            if (!this.querySelector(`[slot=${slotName}]`)) {
-              const slotFactory = this.slots[slotName];
-              const slotContent = slotFactory();
-              if (slotContent instanceof Element) {
-                slotContent.setAttribute('slot', slotName);
-                this.appendChild(slotContent);
-                this.__privateSlots.add(slotName);
-              } // ignore non-elements to enable conditional slots
+    connectedCallback() {
+      if (super.connectedCallback) {
+        super.connectedCallback();
+      }
+      this._connectSlotMixin();
+    }
+
+    _connectSlotMixin() {
+      if (!this.__isConnectedSlotMixin) {
+        Object.keys(this.slots).forEach(slotName => {
+          if (!this.querySelector(`[slot=${slotName}]`)) {
+            const slotFactory = this.slots[slotName];
+            const slotContent = slotFactory();
+            // ignore non-elements to enable conditional slots
+            if (slotContent instanceof Element) {
+              slotContent.setAttribute('slot', slotName);
+              this.appendChild(slotContent);
+              this.__privateSlots.add(slotName);
             }
-          });
-          this.__isConnectedSlotMixin = true;
-        }
+          }
+        });
+        this.__isConnectedSlotMixin = true;
       }
+    }
 
-      /**
-       * @protected
-       * @param {string} slotName Name of the slot
-       * @return {boolean} true if given slot name been created by SlotMixin
-       */
-      _isPrivateSlot(slotName) {
-        return this.__privateSlots.has(slotName);
-      }
-    },
-);
+    /**
+     * @param {string} slotName Name of the slot
+     * @return {boolean} true if given slot name been created by SlotMixin
+     */
+    _isPrivateSlot(slotName) {
+      return this.__privateSlots.has(slotName);
+    }
+  };
+
+export const SlotMixin = dedupeMixin(SlotMixinImplementation);
diff --git a/packages/core/src/UpdateStylesMixin.js b/packages/core/src/UpdateStylesMixin.js
index 3ad4f37e3..753760dd6 100644
--- a/packages/core/src/UpdateStylesMixin.js
+++ b/packages/core/src/UpdateStylesMixin.js
@@ -1,59 +1,78 @@
 /* global ShadyCSS */
 import { dedupeMixin } from '@open-wc/dedupe-mixin';
 
-export const UpdateStylesMixin = dedupeMixin(
-  superclass =>
-    // eslint-disable-next-line no-shadow
-    class UpdateStylesMixin extends superclass {
-      /**
-       * @example
-       * <my-element>
-       *     <style>
-       *         :host {
-       *          color: var(--foo);
-       *         }
-       *     </style>
-       * </my-element>
-       *
-       * $0.updateStyles({'background': 'orange', '--foo': '#fff'})
-       * Chrome, Firefox: <my-element style="background: orange; --foo: bar;">
-       * IE: <my-element>
-       *     => to head: <style>color: #fff</style>
-       *
-       * @param {Object} updateStyles
-       */
-      updateStyles(updateStyles) {
-        const styleString = this.getAttribute('style') || this.getAttribute('data-style') || '';
-        const currentStyles = styleString.split(';').reduce((acc, stylePair) => {
-          const parts = stylePair.split(':');
-          if (parts.length === 2) {
-            /* eslint-disable-next-line prefer-destructuring */
-            acc[parts[0]] = parts[1];
-          }
-          return acc;
-        }, {});
+/**
+ * @typedef {import('../types/UpdateStylesMixinTypes').UpdateStylesMixin} UpdateStylesMixin
+ * @typedef {import('../types/UpdateStylesMixinTypes').StylesMap} StylesMap
+ */
 
-        const newStyles = { ...currentStyles, ...updateStyles };
-        let newStylesString = '';
-        if (typeof ShadyCSS === 'object' && !ShadyCSS.nativeShadow) {
-          // No ShadowDOM => IE, Edge
-          const newCssVariablesObj = {};
-          Object.keys(newStyles).forEach(key => {
-            if (key.indexOf('--') === -1) {
-              newStylesString += `${key}:${newStyles[key]};`;
-            } else {
-              newCssVariablesObj[key] = newStyles[key];
-            }
-          });
-          this.setAttribute('style', newStylesString);
-          ShadyCSS.styleSubtree(this, newCssVariablesObj);
-        } else {
-          // has shadowdom => Chrome, Firefox, Safari
-          Object.keys(newStyles).forEach(key => {
-            newStylesString += `${key}: ${newStyles[key]};`;
-          });
-          this.setAttribute('style', newStylesString);
+/** @type {UpdateStylesMixin} */
+const UpdateStylesMixinImplementation = superclass =>
+  // eslint-disable-next-line no-shadow
+  class UpdateStylesMixinHost extends superclass {
+    /**
+     * @example
+     * <my-element>
+     *     <style>
+     *         :host {
+     *          color: var(--foo);
+     *         }
+     *     </style>
+     * </my-element>
+     *
+     * $0.updateStyles({'background': 'orange', '--foo': '#fff'})
+     * Chrome, Firefox: <my-element style="background: orange; --foo: bar;">
+     * IE: <my-element>
+     *     => to head: <style>color: #fff</style>
+     *
+     * @param {StylesMap} updateStyles
+     */
+    updateStyles(updateStyles) {
+      const styleString = this.getAttribute('style') || this.getAttribute('data-style') || '';
+
+      /**
+       * reducer function
+       * @param {Object.<string, string>} acc
+       * @param {string} stylePair
+       */
+      const reducer = (acc, stylePair) => {
+        /** @type {Array.<string>} */
+        const parts = stylePair.split(':');
+        if (parts.length === 2) {
+          // eslint-disable-next-line prefer-destructuring
+          acc[parts[0]] = parts[1];
         }
+        return acc;
+      };
+      const currentStyles = styleString.split(';').reduce(reducer, {});
+
+      const newStyles = { ...currentStyles, ...updateStyles };
+      let newStylesString = '';
+      // @ts-ignore not sure how to type ShadyCSS..
+      if (typeof ShadyCSS === 'object' && !ShadyCSS.nativeShadow) {
+        // No ShadowDOM => IE, Edge
+
+        /** @type {Object.<string, string>} */
+        const newCssVariablesObj = {};
+
+        Object.keys(newStyles).forEach(key => {
+          if (key.indexOf('--') === -1) {
+            newStylesString += `${key}:${newStyles[key]};`;
+          } else {
+            newCssVariablesObj[key] = newStyles[key];
+          }
+        });
+        this.setAttribute('style', newStylesString);
+        // @ts-ignore not sure how to type ShadyCSS..
+        ShadyCSS.styleSubtree(this, newCssVariablesObj);
+      } else {
+        // has shadowdom => Chrome, Firefox, Safari
+        Object.keys(newStyles).forEach(key => {
+          newStylesString += `${key}: ${newStyles[key]};`;
+        });
+        this.setAttribute('style', newStylesString);
       }
-    },
-);
+    }
+  };
+
+export const UpdateStylesMixin = dedupeMixin(UpdateStylesMixinImplementation);
diff --git a/packages/core/src/differentKeyEventNamesShimIE.js b/packages/core/src/differentKeyEventNamesShimIE.js
index f67a28ae9..c62b5e8c5 100644
--- a/packages/core/src/differentKeyEventNamesShimIE.js
+++ b/packages/core/src/differentKeyEventNamesShimIE.js
@@ -3,6 +3,7 @@ if (typeof window.KeyboardEvent !== 'function') {
   const event = KeyboardEvent.prototype;
   const descriptor = Object.getOwnPropertyDescriptor(event, 'key');
   if (descriptor) {
+    /** @type {Object.<string, string>} */
     const keys = {
       Win: 'Meta',
       Scroll: 'ScrollLock',
@@ -26,10 +27,13 @@ if (typeof window.KeyboardEvent !== 'function') {
     Object.defineProperty(event, 'key', {
       // eslint-disable-next-line object-shorthand, func-names
       get: function () {
-        const key = descriptor.get.call(this);
+        if (descriptor.get) {
+          const key = descriptor.get.call(this);
 
-        // eslint-disable-next-line no-prototype-builtins
-        return keys.hasOwnProperty(key) ? keys[key] : key;
+          // eslint-disable-next-line no-prototype-builtins
+          return keys.hasOwnProperty(key) ? keys[key] : key;
+        }
+        return undefined;
       },
     });
   }
diff --git a/packages/core/test/DelegateMixin.test.js b/packages/core/test/DelegateMixin.test.js
index 09191b15e..a9265bf7e 100644
--- a/packages/core/test/DelegateMixin.test.js
+++ b/packages/core/test/DelegateMixin.test.js
@@ -14,7 +14,7 @@ describe('DelegateMixin', () => {
         get delegations() {
           return {
             ...super.delegations,
-            target: () => this.shadowRoot.getElementById('button1'),
+            target: () => this.shadowRoot?.getElementById('button1'),
             events: ['click'],
           };
         }
@@ -27,7 +27,7 @@ describe('DelegateMixin', () => {
     const element = await fixture(`<${tag}></${tag}>`);
     const cb = sinon.spy();
     element.addEventListener('click', cb);
-    element.shadowRoot.getElementById('button1').click();
+    element.shadowRoot?.getElementById('button1')?.click();
     expect(cb.callCount).to.equal(1);
   });
 
@@ -37,7 +37,7 @@ describe('DelegateMixin', () => {
         get delegations() {
           return {
             ...super.delegations,
-            target: () => this.shadowRoot.getElementById('button1'),
+            target: () => this.shadowRoot?.getElementById('button1'),
             events: ['click'],
           };
         }
@@ -47,12 +47,12 @@ describe('DelegateMixin', () => {
         }
       },
     );
-    const element = document.createElement(tag);
+    const element = /** @type {LitElement} */ (document.createElement(tag));
     const cb = sinon.spy();
     element.addEventListener('click', cb);
     document.body.appendChild(element);
     await element.updateComplete;
-    element.shadowRoot.getElementById('button1').click();
+    element.shadowRoot?.getElementById('button1')?.click();
     expect(cb.callCount).to.equal(1);
 
     // cleanup
@@ -80,37 +80,37 @@ describe('DelegateMixin', () => {
       },
     );
 
-    const element = await fixture(`
-      <${tag}><button slot="button">click me</button></${tag}>`);
+    const element = await fixture(`<${tag}><button slot="button">click me</button></${tag}>`);
     const cb = sinon.spy();
     element.addEventListener('click', cb);
-    Array.from(element.children)
-      .find(child => child.slot === 'button')
-      .click();
+    const childEl = /** @type {HTMLElement} */ (Array.from(element.children)?.find(
+      child => child.slot === 'button',
+    ));
+    childEl?.click();
     expect(cb.callCount).to.equal(1);
   });
 
   it('will still support other events', async () => {
-    const tag = defineCE(
-      class extends DelegateMixin(LitElement) {
-        get delegations() {
-          return {
-            ...super.delegations,
-            target: () => this.shadowRoot.getElementById('button1'),
-            events: ['click'],
-          };
-        }
+    class FooDelegate extends DelegateMixin(LitElement) {
+      get delegations() {
+        return {
+          ...super.delegations,
+          target: () => this.shadowRoot?.getElementById('button1'),
+          events: ['click'],
+        };
+      }
 
-        render() {
-          return html`<button id="button1">with delegation</button>`;
-        }
+      render() {
+        return html`<button id="button1">with delegation</button>`;
+      }
 
-        foo() {
-          this.dispatchEvent(new CustomEvent('foo-event', { bubbles: true, composed: true }));
-        }
-      },
-    );
-    const element = await fixture(`<${tag}></${tag}>`);
+      foo() {
+        this.dispatchEvent(new CustomEvent('foo-event', { bubbles: true, composed: true }));
+      }
+    }
+
+    const tag = defineCE(FooDelegate);
+    const element = /** @type {FooDelegate} */ (await fixture(`<${tag}></${tag}>`));
     const cb = sinon.spy();
     element.addEventListener('foo-event', cb);
     element.foo();
@@ -123,7 +123,7 @@ describe('DelegateMixin', () => {
         get delegations() {
           return {
             ...super.delegations,
-            target: () => this.shadowRoot.getElementById('button1'),
+            target: () => this.shadowRoot?.getElementById('button1'),
             methods: ['click'],
           };
         }
@@ -133,9 +133,9 @@ describe('DelegateMixin', () => {
         }
       },
     );
-    const element = await fixture(`<${tag}></${tag}>`);
+    const element = /** @type {HTMLElement} */ (await fixture(`<${tag}></${tag}>`));
     const cb = sinon.spy();
-    element.shadowRoot.getElementById('button1').addEventListener('click', cb);
+    element.shadowRoot?.getElementById('button1')?.addEventListener('click', cb);
     element.click();
     expect(cb.callCount).to.equal(1);
   });
@@ -147,56 +147,69 @@ describe('DelegateMixin', () => {
         this.foo = { a: 'a', b: 'b' };
       }
 
+      /**
+       * @param {?} a
+       * @param {?} b
+       */
       setFooAandB(a, b) {
         this.foo.a = a;
         this.foo.b = b;
       }
     }
     customElements.define('delegate-argument-sub', DelegateArgumentSub);
-    const tag = defineCE(
-      class extends DelegateMixin(LitElement) {
-        get delegations() {
-          return {
-            ...super.delegations,
-            target: () => this.shadowRoot.getElementById('sub'),
-            methods: ['setFooAandB'],
-          };
-        }
 
-        render() {
-          return html`<delegate-argument-sub id="sub"></delegate-argument-sub>`;
-        }
-      },
-    );
+    class DelegateArgumentParent extends DelegateMixin(LitElement) {
+      get delegations() {
+        return {
+          ...super.delegations,
+          target: () => this.shadowRoot?.getElementById('sub'),
+          methods: ['setFooAandB'],
+        };
+      }
 
-    const element = await fixture(`<${tag}></${tag}>`);
-    element.disabled = true;
+      render() {
+        return html`<delegate-argument-sub id="sub"></delegate-argument-sub>`;
+      }
+    }
+    const tag = defineCE(DelegateArgumentParent);
+
+    const element = /** @type {DelegateArgumentParent} */ (await fixture(`<${tag}></${tag}>`));
+
+    // @ts-ignore because this method, even though it doesn't exist on the parent, gets delegated through delegations to the child, where it does exist!
     element.setFooAandB('newA', 'newB');
-    expect(element.shadowRoot.getElementById('sub').foo.a).to.equal('newA');
-    expect(element.shadowRoot.getElementById('sub').foo.b).to.equal('newB');
+
+    const sub = /** @type {DelegateArgumentSub} */ (element.shadowRoot?.getElementById('sub'));
+    expect(sub.foo.a).to.equal('newA');
+    expect(sub.foo.b).to.equal('newB');
   });
 
   it('will set delegated properties', async () => {
-    const tag = defineCE(
-      class extends DelegateMixin(LitElement) {
-        get delegations() {
-          return {
-            ...super.delegations,
-            target: () => this.shadowRoot.getElementById('button1'),
-            properties: ['disabled'],
-          };
-        }
+    class PropDelegate extends DelegateMixin(LitElement) {
+      get delegations() {
+        return {
+          ...super.delegations,
+          target: () => this.shadowRoot?.getElementById('button1'),
+          properties: ['disabled'],
+        };
+      }
 
-        render() {
-          return html`<button id="button1">with delegation</button>`;
-        }
-      },
-    );
-    const element = await fixture(`<${tag}></${tag}>`);
+      render() {
+        return html`<button id="button1">with delegation</button>`;
+      }
+    }
+    const tag = defineCE(PropDelegate);
+    const element = /** @type {PropDelegate} */ (await fixture(`<${tag}></${tag}>`));
+
+    // @ts-ignore ignoring this one, because disabled is delegated through target so it indeed does not inherently exist on the div element
     element.disabled = true;
+
     await element.updateComplete;
-    expect(element.shadowRoot.getElementById('button1').disabled).to.equal(true);
-    expect(element.shadowRoot.getElementById('button1').hasAttribute('disabled')).to.equal(true);
+
+    /** @typedef {Object.<string,boolean>} Btn */
+    /** @typedef {Btn & HTMLElement} DelegatedBtn */
+    const btn = /** @type {DelegatedBtn} */ (element.shadowRoot?.getElementById('button1'));
+    expect(btn?.disabled).to.equal(true);
+    expect(btn?.hasAttribute('disabled')).to.equal(true);
   });
 
   it('delegates properties before delegation target is attached to DOM', async () => {
@@ -205,7 +218,7 @@ describe('DelegateMixin', () => {
         get delegations() {
           return {
             ...super.delegations,
-            target: () => this.shadowRoot.getElementById('button1'),
+            target: () => this.shadowRoot?.getElementById('button1'),
             properties: ['disabled'],
           };
         }
@@ -215,12 +228,18 @@ describe('DelegateMixin', () => {
         }
       },
     );
-    const element = document.createElement(tag);
+    /** @typedef {Object.<string,boolean>} Btn */
+    /** @typedef {Btn & LitElement} DelegatedEl */
+    const element = /** @type {DelegatedEl} */ (document.createElement(tag));
+
     element.disabled = true;
     document.body.appendChild(element);
     await element.updateComplete;
-    expect(element.shadowRoot.getElementById('button1').disabled).to.equal(true);
 
+    /** @typedef {Btn & HTMLElement} DelegatedBtn */
+    const btn = /** @type {DelegatedBtn} */ (element.shadowRoot?.getElementById('button1'));
+
+    expect(btn?.disabled).to.equal(true);
     // cleanup
     document.body.removeChild(element);
   });
@@ -231,7 +250,7 @@ describe('DelegateMixin', () => {
         get delegations() {
           return {
             ...super.delegations,
-            target: () => this.shadowRoot.getElementById('button1'),
+            target: () => this.shadowRoot?.getElementById('button1'),
             attributes: ['disabled'],
           };
         }
@@ -241,11 +260,11 @@ describe('DelegateMixin', () => {
         }
       },
     );
-    const element = await fixture(`<${tag}></${tag}>`);
+    const element = /** @type {LitElement} */ (await fixture(`<${tag}></${tag}>`));
     element.setAttribute('disabled', '');
     await element.updateComplete;
     expect(element.hasAttribute('disabled')).to.equal(false);
-    expect(element.shadowRoot.getElementById('button1').hasAttribute('disabled')).to.equal(true);
+    expect(element.shadowRoot?.getElementById('button1')?.hasAttribute('disabled')).to.equal(true);
   });
 
   it('will read inital attributes', async () => {
@@ -254,7 +273,7 @@ describe('DelegateMixin', () => {
         get delegations() {
           return {
             ...super.delegations,
-            target: () => this.shadowRoot.getElementById('button1'),
+            target: () => this.shadowRoot?.getElementById('button1'),
             attributes: ['disabled'],
           };
         }
@@ -266,7 +285,7 @@ describe('DelegateMixin', () => {
     );
     const element = await fixture(`<${tag} disabled></${tag}>`);
     expect(element.hasAttribute('disabled')).to.equal(false);
-    expect(element.shadowRoot.getElementById('button1').hasAttribute('disabled')).to.equal(true);
+    expect(element.shadowRoot?.getElementById('button1')?.hasAttribute('disabled')).to.equal(true);
   });
 
   it('will delegate removeAttribute', async () => {
@@ -275,7 +294,7 @@ describe('DelegateMixin', () => {
         get delegations() {
           return {
             ...super.delegations,
-            target: () => this.shadowRoot.getElementById('button1'),
+            target: () => this.shadowRoot?.getElementById('button1'),
             attributes: ['disabled'],
           };
         }
@@ -285,135 +304,146 @@ describe('DelegateMixin', () => {
         }
       },
     );
-    const element = await fixture(`<${tag} disabled></${tag}>`);
-    element.removeAttribute('disabled', '');
+    const element = /** @type {LitElement} */ (await fixture(`<${tag} disabled></${tag}>`));
+    element.removeAttribute('disabled');
     await element.updateComplete;
     expect(element.hasAttribute('disabled')).to.equal(false);
-    expect(element.shadowRoot.getElementById('button1').hasAttribute('disabled')).to.equal(false);
+    expect(element.shadowRoot?.getElementById('button1')?.hasAttribute('disabled')).to.equal(false);
   });
 
   it('respects user defined values for delegated attributes and properties', async () => {
-    const tag = defineCE(
-      class extends DelegateMixin(LitElement) {
-        get delegations() {
-          return {
-            ...super.delegations,
-            // this just means itś config is set to the queue when called before connectedCallback
-            target: () => this.scheduledElement,
-            attributes: ['type'],
-            properties: ['type'],
-          };
-        }
+    class ScheduledElement extends DelegateMixin(LitElement) {
+      get delegations() {
+        return {
+          ...super.delegations,
+          // this just means itś config is set to the queue when called before connectedCallback
+          target: () => this.scheduledElement,
+          attributes: ['type'],
+          properties: ['type'],
+        };
+      }
 
-        get scheduledElement() {
-          return this.querySelector('input');
-        }
+      get scheduledElement() {
+        return this.querySelector('input');
+      }
 
-        constructor() {
-          super();
-          this.type = 'email'; // 1. here we set the delegated prop and it should be scheduled
-        }
+      constructor() {
+        super();
+        this.type = 'email'; // 1. here we set the delegated prop and it should be scheduled
+      }
 
-        connectedCallback() {
-          // 2. this is where we add teh delegation target (so after 1)
-          this.appendChild(document.createElement('input'));
-          super.connectedCallback(); // let the DelegateMixin do its work
-        }
-      },
-    );
+      connectedCallback() {
+        // 2. this is where we add teh delegation target (so after 1)
+        this.appendChild(document.createElement('input'));
+        super.connectedCallback(); // let the DelegateMixin do its work
+      }
+    }
+
+    const tag = defineCE(ScheduledElement);
     const tagName = unsafeStatic(tag);
+
     // Here, the Application Developerd tries to set the type via attribute
-    const elementAttr = await fixture(`<${tag} type="radio"></${tag}>`);
-    expect(elementAttr.scheduledElement.type).to.equal('radio');
+    const elementAttr = /** @type {ScheduledElement} */ (await fixture(
+      `<${tag} type="radio"></${tag}>`,
+    ));
+    expect(elementAttr.scheduledElement?.type).to.equal('radio');
     // Here, the Application Developer tries to set the type via property
-    const elementProp = await fixture(html`<${tagName} .type=${'radio'}></${tagName}>`);
-    expect(elementProp.scheduledElement.type).to.equal('radio');
+    const elementProp = /** @type {ScheduledElement} */ (await fixture(
+      html`<${tagName} .type=${'radio'}></${tagName}>`,
+    ));
+    expect(elementProp.scheduledElement?.type).to.equal('radio');
   });
 
   it(`uses attribute value as a fallback for delegated property getter
     when property not set by user and delegationTarget not ready`, async () => {
-    const tag = defineCE(
-      class extends DelegateMixin(LitElement) {
-        get delegations() {
-          return {
-            ...super.delegations,
-            target: () => this.delegatedEl,
-            properties: ['type'],
-            attributes: ['type'],
-          };
-        }
+    class FallbackEl extends DelegateMixin(LitElement) {
+      get delegations() {
+        return {
+          ...super.delegations,
+          target: () => this.delegatedEl,
+          properties: ['type'],
+          attributes: ['type'],
+        };
+      }
 
-        get delegatedEl() {
-          // returns null, so we can test that "cached" attr is used as fallback
-          return null;
-        }
-      },
-    );
-    const element = await fixture(`<${tag} type="radio"></${tag}>`);
+      get delegatedEl() {
+        // returns null, so we can test that "cached" attr is used as fallback
+        return null;
+      }
+    }
+    const tag = defineCE(FallbackEl);
+    const element = /** @type {FallbackEl} */ (await fixture(`<${tag} type="radio"></${tag}>`));
     expect(element.delegatedEl).to.equal(null);
+    // @ts-ignore ignoring this one, because type is delegated through target so it indeed does not inherently exist on the div element
     expect(element.type).to.equal('radio'); // value retrieved from host instead of delegatedTarget
   });
 
   it('works with connectedCallback', async () => {
-    const tag = await defineCE(
-      class extends DelegateMixin(HTMLElement) {
-        get delegations() {
-          return {
-            ...super.delegations,
-            target: () => this.querySelector('div'),
-            properties: ['foo'],
-          };
-        }
-      },
-    );
-    const element = await fixture(`<${tag}><div></div></${tag}>`);
+    class ConnectedElement extends DelegateMixin(HTMLElement) {
+      get delegations() {
+        return {
+          ...super.delegations,
+          target: () => this.querySelector('div'),
+          properties: ['foo'],
+        };
+      }
+    }
+    const tag = await defineCE(ConnectedElement);
+    const element = /** @type {ConnectedElement} */ (await fixture(`<${tag}><div></div></${tag}>`));
+
+    // @ts-ignore ignoring this one, because foo is delegated through target so it indeed does not inherently exist on the div element
     element.foo = 'new';
-    expect(element.querySelector('div').foo).to.equal('new');
+    // @ts-ignore ignoring this one, because foo is delegated through target so it indeed does not inherently exist on the div element
+    expect(element.querySelector('div')?.foo).to.equal('new');
   });
 
   it('works with shadow dom', async () => {
-    const tag = await defineCE(
-      class extends DelegateMixin(LitElement) {
-        get delegations() {
-          return {
-            ...super.delegations,
-            target: () => this.shadowRoot.querySelector('div'),
-            properties: ['foo'],
-          };
-        }
+    class A extends DelegateMixin(LitElement) {
+      get delegations() {
+        return {
+          ...super.delegations,
+          target: () => this.shadowRoot?.querySelector('div'),
+          properties: ['foo'],
+        };
+      }
 
-        render() {
-          return html`<div></div>`;
-        }
-      },
-    );
+      render() {
+        return html`<div></div>`;
+      }
+    }
+    const tag = await defineCE(A);
     const element = await fixture(`<${tag}></${tag}>`);
+
+    // @ts-ignore ignoring this one, because foo is delegated through target so it indeed does not inherently exist on the div element
     element.foo = 'new';
-    expect(element.shadowRoot.querySelector('div').foo).to.equal('new');
+    // @ts-ignore ignoring this one, because foo is delegated through target so it indeed does not inherently exist on the div element
+    expect(element.shadowRoot?.querySelector('div')?.foo).to.equal('new');
   });
 
   it('works with light dom', async () => {
-    const tag = await defineCE(
-      class extends DelegateMixin(LitElement) {
-        get delegations() {
-          return {
-            ...super.delegations,
-            target: () => this.querySelector('div'),
-            properties: ['foo'],
-          };
-        }
+    class A extends DelegateMixin(LitElement) {
+      get delegations() {
+        return {
+          ...super.delegations,
+          target: () => this.querySelector('div'),
+          properties: ['foo'],
+        };
+      }
 
-        createRenderRoot() {
-          return this;
-        }
+      createRenderRoot() {
+        return this;
+      }
 
-        render() {
-          return html`<div></div>`;
-        }
-      },
-    );
+      render() {
+        return html`<div></div>`;
+      }
+    }
+
+    const tag = await defineCE(A);
     const element = await fixture(`<${tag}></${tag}>`);
+    // @ts-ignore ignoring this one, because foo is delegated through target so it indeed does not inherently exist on the div element
     element.foo = 'new';
-    expect(element.querySelector('div').foo).to.equal('new');
+    // @ts-ignore ignoring this one, because foo is delegated through target so it indeed does not inherently exist on the div element
+    expect(element.querySelector('div')?.foo).to.equal('new');
   });
 });
diff --git a/packages/core/test/DisabledMixin.test.js b/packages/core/test/DisabledMixin.test.js
index c9bcf11a7..52f1bc9cc 100644
--- a/packages/core/test/DisabledMixin.test.js
+++ b/packages/core/test/DisabledMixin.test.js
@@ -3,21 +3,26 @@ import { LitElement } from '../index.js';
 import { DisabledMixin } from '../src/DisabledMixin.js';
 
 describe('DisabledMixin', () => {
+  class CanBeDisabled extends DisabledMixin(LitElement) {}
   before(() => {
-    class CanBeDisabled extends DisabledMixin(LitElement) {}
     customElements.define('can-be-disabled', CanBeDisabled);
   });
 
   it('reflects disabled to attribute', async () => {
-    const el = await fixture(html`<can-be-disabled></can-be-disabled>`);
+    const el = /** @type {CanBeDisabled} */ (await fixture(
+      html`<can-be-disabled></can-be-disabled>`,
+    ));
     expect(el.hasAttribute('disabled')).to.be.false;
+    el.makeRequestToBeDisabled();
     el.disabled = true;
     await el.updateComplete;
     expect(el.hasAttribute('disabled')).to.be.true;
   });
 
   it('can be requested to be disabled', async () => {
-    const el = await fixture(html`<can-be-disabled></can-be-disabled>`);
+    const el = /** @type {CanBeDisabled} */ (await fixture(
+      html`<can-be-disabled></can-be-disabled>`,
+    ));
     el.makeRequestToBeDisabled();
     expect(el.disabled).to.be.true;
     await el.updateComplete;
@@ -25,7 +30,9 @@ describe('DisabledMixin', () => {
   });
 
   it('will not allow to become enabled after makeRequestToBeDisabled()', async () => {
-    const el = await fixture(html`<can-be-disabled></can-be-disabled>`);
+    const el = /** @type {CanBeDisabled} */ (await fixture(
+      html`<can-be-disabled></can-be-disabled>`,
+    ));
     el.makeRequestToBeDisabled();
     expect(el.disabled).to.be.true;
 
@@ -34,14 +41,18 @@ describe('DisabledMixin', () => {
   });
 
   it('will stay disabled after retractRequestToBeDisabled() if it was disabled before', async () => {
-    const el = await fixture(html`<can-be-disabled disabled></can-be-disabled>`);
+    const el = /** @type {CanBeDisabled} */ (await fixture(
+      html`<can-be-disabled disabled></can-be-disabled>`,
+    ));
     el.makeRequestToBeDisabled();
     el.retractRequestToBeDisabled();
     expect(el.disabled).to.be.true;
   });
 
   it('will become enabled after retractRequestToBeDisabled() if it was enabled before', async () => {
-    const el = await fixture(html`<can-be-disabled></can-be-disabled>`);
+    const el = /** @type {CanBeDisabled} */ (await fixture(
+      html`<can-be-disabled></can-be-disabled>`,
+    ));
     el.makeRequestToBeDisabled();
     expect(el.disabled).to.be.true;
     el.retractRequestToBeDisabled();
@@ -49,7 +60,9 @@ describe('DisabledMixin', () => {
   });
 
   it('may allow multiple calls to makeRequestToBeDisabled()', async () => {
-    const el = await fixture(html`<can-be-disabled></can-be-disabled>`);
+    const el = /** @type {CanBeDisabled} */ (await fixture(
+      html`<can-be-disabled></can-be-disabled>`,
+    ));
     el.makeRequestToBeDisabled();
     el.makeRequestToBeDisabled();
     el.retractRequestToBeDisabled();
@@ -57,7 +70,9 @@ describe('DisabledMixin', () => {
   });
 
   it('will restore last state after retractRequestToBeDisabled()', async () => {
-    const el = await fixture(html`<can-be-disabled></can-be-disabled>`);
+    const el = /** @type {CanBeDisabled} */ (await fixture(
+      html`<can-be-disabled></can-be-disabled>`,
+    ));
     el.makeRequestToBeDisabled();
     el.disabled = true;
     el.retractRequestToBeDisabled();
diff --git a/packages/core/test/DisabledWithTabIndexMixin.test.js b/packages/core/test/DisabledWithTabIndexMixin.test.js
index 3a037f8bc..4afd740da 100644
--- a/packages/core/test/DisabledWithTabIndexMixin.test.js
+++ b/packages/core/test/DisabledWithTabIndexMixin.test.js
@@ -4,23 +4,23 @@ import { LitElement } from '../index.js';
 import { DisabledWithTabIndexMixin } from '../src/DisabledWithTabIndexMixin.js';
 
 describe('DisabledWithTabIndexMixin', () => {
+  class WithTabIndex extends DisabledWithTabIndexMixin(LitElement) {}
   before(() => {
-    class WithTabIndex extends DisabledWithTabIndexMixin(LitElement) {}
     customElements.define('can-be-disabled-with-tab-index', WithTabIndex);
   });
 
   it('has an initial tabIndex of 0', async () => {
-    const el = await fixture(html`
+    const el = /** @type {WithTabIndex} */ (await fixture(html`
       <can-be-disabled-with-tab-index></can-be-disabled-with-tab-index>
-    `);
+    `));
     expect(el.tabIndex).to.equal(0);
     expect(el.getAttribute('tabindex')).to.equal('0');
   });
 
   it('sets tabIndex to -1 if disabled', async () => {
-    const el = await fixture(html`
+    const el = /** @type {WithTabIndex} */ (await fixture(html`
       <can-be-disabled-with-tab-index></can-be-disabled-with-tab-index>
-    `);
+    `));
     el.disabled = true;
     expect(el.tabIndex).to.equal(-1);
     await el.updateComplete;
@@ -28,9 +28,9 @@ describe('DisabledWithTabIndexMixin', () => {
   });
 
   it('disabled does not override user provided tabindex', async () => {
-    const el = await fixture(html`
+    const el = /** @type {WithTabIndex} */ (await fixture(html`
       <can-be-disabled-with-tab-index tabindex="5" disabled></can-be-disabled-with-tab-index>
-    `);
+    `));
     expect(el.getAttribute('tabindex')).to.equal('-1');
     el.disabled = false;
     await el.updateComplete;
@@ -38,9 +38,9 @@ describe('DisabledWithTabIndexMixin', () => {
   });
 
   it('can be disabled imperatively', async () => {
-    const el = await fixture(html`
+    const el = /** @type {WithTabIndex} */ (await fixture(html`
       <can-be-disabled-with-tab-index disabled></can-be-disabled-with-tab-index>
-    `);
+    `));
     expect(el.getAttribute('tabindex')).to.equal('-1');
 
     el.disabled = false;
@@ -55,9 +55,9 @@ describe('DisabledWithTabIndexMixin', () => {
   });
 
   it('will not allow to change tabIndex after makeRequestToBeDisabled()', async () => {
-    const el = await fixture(html`
+    const el = /** @type {WithTabIndex} */ (await fixture(html`
       <can-be-disabled-with-tab-index></can-be-disabled-with-tab-index>
-    `);
+    `));
     el.makeRequestToBeDisabled();
 
     el.tabIndex = 5;
@@ -67,9 +67,9 @@ describe('DisabledWithTabIndexMixin', () => {
   });
 
   it('will restore last tabIndex after retractRequestToBeDisabled()', async () => {
-    const el = await fixture(html`
+    const el = /** @type {WithTabIndex} */ (await fixture(html`
       <can-be-disabled-with-tab-index tabindex="5"></can-be-disabled-with-tab-index>
-    `);
+    `));
     el.makeRequestToBeDisabled();
     expect(el.tabIndex).to.equal(-1);
     await el.updateComplete;
@@ -96,9 +96,9 @@ describe('DisabledWithTabIndexMixin', () => {
   });
 
   it('may allow multiple calls to retractRequestToBeDisabled', async () => {
-    const el = await fixture(html`
+    const el = /** @type {WithTabIndex} */ (await fixture(html`
       <can-be-disabled-with-tab-index disabled></can-be-disabled-with-tab-index>
-    `);
+    `));
     el.retractRequestToBeDisabled();
     el.retractRequestToBeDisabled();
     expect(el.disabled).to.be.true;
diff --git a/packages/core/test/LionSingleton.test.js b/packages/core/test/LionSingleton.test.js
index b6208fb3b..27d808294 100644
--- a/packages/core/test/LionSingleton.test.js
+++ b/packages/core/test/LionSingleton.test.js
@@ -11,6 +11,11 @@ describe('LionSingleton', () => {
 
   it('supports parameters for .getInstance(foo, bar)', async () => {
     class MySingleton extends LionSingleton {
+      /**
+       *
+       * @param {string} foo
+       * @param {string} bar
+       */
       constructor(foo, bar) {
         super();
         this.foo = foo;
@@ -59,6 +64,7 @@ describe('LionSingleton', () => {
   });
 
   it('can at any time add mixins via .addInstanceMixin()', () => {
+    // @ts-ignore because we're getting rid of LionSingleton altogether
     const MyMixin = superclass =>
       class extends superclass {
         constructor() {
@@ -72,6 +78,7 @@ describe('LionSingleton', () => {
     const mySingleton = MySingleton.getInstance();
     expect(mySingleton.myMixin).to.be.true;
 
+    // @ts-ignore because we're getting rid of LionSingleton altogether
     const OtherMixin = superclass =>
       class extends superclass {
         constructor() {
@@ -89,6 +96,7 @@ describe('LionSingleton', () => {
   });
 
   it('can provide new instances (with applied Mixins) via .getNewInstance()', async () => {
+    // @ts-ignore because we're getting rid of LionSingleton altogether
     const MyMixin = superclass =>
       class extends superclass {
         constructor() {
@@ -100,13 +108,19 @@ describe('LionSingleton', () => {
 
     MySingleton.addInstanceMixin(MyMixin);
     const singletonOne = MySingleton.getNewInstance();
+    // @ts-ignore because we're getting rid of LionSingleton altogether
     singletonOne.one = true;
+    // @ts-ignore because we're getting rid of LionSingleton altogether
     expect(singletonOne.myMixin).to.be.true;
+    // @ts-ignore because we're getting rid of LionSingleton altogether
     expect(singletonOne.one).to.be.true;
 
     const singletonTwo = MySingleton.getNewInstance();
+    // @ts-ignore because we're getting rid of LionSingleton altogether
     expect(singletonTwo.myMixin).to.be.true;
+    // @ts-ignore because we're getting rid of LionSingleton altogether
     expect(singletonTwo.one).to.be.undefined;
+    // @ts-ignore because we're getting rid of LionSingleton altogether
     expect(singletonOne.one).to.be.true; // to be sure
   });
 });
diff --git a/packages/core/test/SlotMixin.test.js b/packages/core/test/SlotMixin.test.js
index 0ff42feba..675c875ca 100644
--- a/packages/core/test/SlotMixin.test.js
+++ b/packages/core/test/SlotMixin.test.js
@@ -14,8 +14,8 @@ describe('SlotMixin', () => {
         }
       },
     );
-    const element = await fixture(`<${tag}></${tag}>`);
-    expect(element.children[0].slot).to.equal('feedback');
+    const el = await fixture(`<${tag}></${tag}>`);
+    expect(el.children[0].slot).to.equal('feedback');
   });
 
   it('does not override user provided slots', async () => {
@@ -31,7 +31,7 @@ describe('SlotMixin', () => {
     );
     const el = await fixture(`<${tag}><p slot="feedback">user-content</p></${tag}>`);
     expect(el.children[0].tagName).to.equal('P');
-    expect(el.children[0].innerText).to.equal('user-content');
+    expect(/** @type HTMLParagraphElement */ (el.children[0]).innerText).to.equal('user-content');
   });
 
   it('supports complex dom trees as element', async () => {
@@ -57,10 +57,12 @@ describe('SlotMixin', () => {
         }
       },
     );
-    const element = await fixture(`<${tag}></${tag}>`);
-    expect(element.children[0].slot).to.equal('feedback');
-    expect(element.children[0].getAttribute('foo')).to.equal('bar');
-    expect(element.children[0].children[0].innerText).to.equal('cat');
+    const el = await fixture(`<${tag}></${tag}>`);
+    expect(el.children[0].slot).to.equal('feedback');
+    expect(el.children[0].getAttribute('foo')).to.equal('bar');
+    expect(/** @type HTMLParagraphElement */ (el.children[0].children[0]).innerText).to.equal(
+      'cat',
+    );
   });
 
   it('supports conditional slots', async () => {
@@ -82,35 +84,37 @@ describe('SlotMixin', () => {
         }
       },
     );
-    const elementSlot = await fixture(`<${tag}><${tag}>`);
-    expect(elementSlot.querySelector('#someSlot')).to.exist;
+    const elSlot = await fixture(`<${tag}><${tag}>`);
+    expect(elSlot.querySelector('#someSlot')).to.exist;
     renderSlot = false;
-    const elementNoSlot = await fixture(`<${tag}><${tag}>`);
-    expect(elementNoSlot.querySelector('#someSlot')).to.not.exist;
+    const elNoSlot = await fixture(`<${tag}><${tag}>`);
+    expect(elNoSlot.querySelector('#someSlot')).to.not.exist;
   });
 
   it("allows to check which slots have been created via this._isPrivateSlot('slotname')", async () => {
     let renderSlot = true;
-    const tag = defineCE(
-      class extends SlotMixin(HTMLElement) {
-        get slots() {
-          return {
-            ...super.slots,
-            conditional: () => (renderSlot ? document.createElement('div') : undefined),
-          };
-        }
+    class SlotPrivateText extends SlotMixin(HTMLElement) {
+      get slots() {
+        return {
+          ...super.slots,
+          conditional: () => (renderSlot ? document.createElement('div') : undefined),
+        };
+      }
 
-        didCreateConditionalSlot() {
-          return this._isPrivateSlot('conditional');
-        }
-      },
-    );
-    const el = await fixture(`<${tag}><${tag}>`);
+      didCreateConditionalSlot() {
+        return this._isPrivateSlot('conditional');
+      }
+    }
+
+    const tag = defineCE(SlotPrivateText);
+    const el = /** @type {SlotPrivateText} */ (await fixture(`<${tag}><${tag}>`));
     expect(el.didCreateConditionalSlot()).to.be.true;
-    const elUserSlot = await fixture(`<${tag}><p slot="conditional">foo</p><${tag}>`);
+    const elUserSlot = /** @type {SlotPrivateText} */ (await fixture(
+      `<${tag}><p slot="conditional">foo</p><${tag}>`,
+    ));
     expect(elUserSlot.didCreateConditionalSlot()).to.be.false;
     renderSlot = false;
-    const elNoSlot = await fixture(`<${tag}><${tag}>`);
+    const elNoSlot = /** @type {SlotPrivateText} */ (await fixture(`<${tag}><${tag}>`));
     expect(elNoSlot.didCreateConditionalSlot()).to.be.false;
   });
 });
diff --git a/packages/core/test/UpdateStylesMixin.test.js b/packages/core/test/UpdateStylesMixin.test.js
index c8c99f307..8c2dc3843 100644
--- a/packages/core/test/UpdateStylesMixin.test.js
+++ b/packages/core/test/UpdateStylesMixin.test.js
@@ -4,82 +4,75 @@ import { UpdateStylesMixin } from '../src/UpdateStylesMixin.js';
 
 describe('UpdateStylesMixin', () => {
   it('handles css variables && direct e.g. host css properties correctly', async () => {
-    const tag = defineCE(
-      class extends UpdateStylesMixin(LitElement) {
-        static get styles() {
-          return [
-            css`
-              :host {
-                text-align: right;
+    class UpdateStylesElement extends UpdateStylesMixin(LitElement) {
+      static get styles() {
+        return [
+          css`
+            :host {
+              text-align: right;
 
-                --color: rgb(128, 128, 128);
-              }
+              --color: rgb(128, 128, 128);
+            }
 
-              h1 {
-                color: var(--color);
-              }
-            `,
-          ];
-        }
+            h1 {
+              color: var(--color);
+            }
+          `,
+        ];
+      }
 
-        render() {
-          return html`<h1 id="header">hey</h1>`;
-        }
-      },
-    );
-    const el = await fixture(`<${tag}></${tag}>`);
-    expect(window.getComputedStyle(el.shadowRoot.getElementById('header')).color).to.equal(
-      'rgb(128, 128, 128)',
-    );
+      render() {
+        return html`<h1 id="header">hey</h1>`;
+      }
+    }
+
+    const tag = defineCE(UpdateStylesElement);
+    const el = /** @type {UpdateStylesElement} */ (await fixture(`<${tag}></${tag}>`));
+    const header = /** @type {Element} */ (el.shadowRoot?.getElementById('header'));
+
+    expect(window.getComputedStyle(header).color).to.equal('rgb(128, 128, 128)');
     expect(window.getComputedStyle(el).textAlign).to.equal('right');
     el.updateStyles({
       '--color': 'rgb(255, 0, 0)',
       'text-align': 'center',
     });
 
-    await tag.updateComplete;
-    expect(window.getComputedStyle(el.shadowRoot.getElementById('header')).color).to.equal(
-      'rgb(255, 0, 0)',
-    );
+    await el.updateComplete;
+    expect(window.getComputedStyle(header).color).to.equal('rgb(255, 0, 0)');
     expect(window.getComputedStyle(el).textAlign).to.equal('center');
   });
 
   it('preserves existing styles', async () => {
-    const tag = defineCE(
-      class extends UpdateStylesMixin(LitElement) {
-        static get styles() {
-          return [
-            css`
-              :host {
-                --color: rgb(128, 128, 128);
-              }
+    class UpdateStylesElement extends UpdateStylesMixin(LitElement) {
+      static get styles() {
+        return [
+          css`
+            :host {
+              --color: rgb(128, 128, 128);
+            }
 
-              h1 {
-                color: var(--color);
-              }
-            `,
-          ];
-        }
+            h1 {
+              color: var(--color);
+            }
+          `,
+        ];
+      }
 
-        render() {
-          return html`<h1 id="header">hey</h1>`;
-        }
-      },
-    );
-    const el = await fixture(`<${tag}></${tag}>`);
-    expect(window.getComputedStyle(el.shadowRoot.getElementById('header')).color).to.equal(
-      'rgb(128, 128, 128)',
-    );
+      render() {
+        return html`<h1 id="header">hey</h1>`;
+      }
+    }
+    const tag = defineCE(UpdateStylesElement);
+    const el = /** @type {UpdateStylesElement} */ (await fixture(`<${tag}></${tag}>`));
+    const header = /** @type {Element} */ (el.shadowRoot?.getElementById('header'));
+
+    expect(window.getComputedStyle(header).color).to.equal('rgb(128, 128, 128)');
     el.updateStyles({ '--color': 'rgb(255, 0, 0)' });
 
-    await tag.updateComplete;
-    expect(window.getComputedStyle(el.shadowRoot.getElementById('header')).color).to.equal(
-      'rgb(255, 0, 0)',
-    );
+    expect(window.getComputedStyle(header).color).to.equal('rgb(255, 0, 0)');
     el.updateStyles({ 'text-align': 'left' });
 
-    await tag.updateComplete;
-    const styles = window.getComputedStyle(el.shadowRoot.getElementById('header'));
+    const styles = window.getComputedStyle(header);
     expect(styles.color).to.equal('rgb(255, 0, 0)');
     expect(styles.textAlign).to.equal('left');
   });
diff --git a/packages/core/types/DelegateMixinTypes.d.ts b/packages/core/types/DelegateMixinTypes.d.ts
new file mode 100644
index 000000000..0840c3a64
--- /dev/null
+++ b/packages/core/types/DelegateMixinTypes.d.ts
@@ -0,0 +1,51 @@
+import { Constructor } from '@open-wc/dedupe-mixin';
+
+export type Delegations = {
+  target: Function;
+  events: string[];
+  methods: string[];
+  properties: string[];
+  attributes: string[];
+};
+
+export declare class DelegateMixinHost {
+  delegations: Delegations;
+
+  protected _connectDelegateMixin(): void;
+
+  private __setupPropertyDelegation(): void;
+
+  private __initialAttributeDelegation(): void;
+
+  private __emptyEventListenerQueue(): void;
+
+  private __emptyPropertiesQueue(): void;
+}
+
+/**
+ * # DelegateMixin
+ * Forwards defined events, methods, properties and attributes to the defined target.
+ *
+ * @example
+ * get delegations() {
+ *   return {
+ *     ...super.delegations,
+ *     target: () => this.shadowRoot.getElementById('button1'),
+ *     events: ['click'],
+ *     methods: ['click'],
+ *     properties: ['disabled'],
+ *     attributes: ['disabled'],
+ *   };
+ * }
+ *
+ * render() {
+ *   return html`
+ *     <button id="button1">with delegation</button>
+ *   `;
+ * }
+ */
+declare function DelegateMixinImplementation<T extends Constructor<HTMLElement>>(
+  superclass: T,
+): T & Constructor<DelegateMixinHost>;
+
+export type DelegateMixin = typeof DelegateMixinImplementation;
diff --git a/packages/core/types/DisabledMixinTypes.d.ts b/packages/core/types/DisabledMixinTypes.d.ts
new file mode 100644
index 000000000..ade1a1178
--- /dev/null
+++ b/packages/core/types/DisabledMixinTypes.d.ts
@@ -0,0 +1,29 @@
+import { Constructor } from '@open-wc/dedupe-mixin';
+
+export declare class DisabledMixinHost {
+  static get properties(): {
+    disabled: {
+      type: BooleanConstructor;
+      reflect: boolean;
+    };
+  };
+  disabled: boolean;
+
+  /**
+   * Makes request to make the element disabled
+   */
+  public makeRequestToBeDisabled(): void;
+
+  /**
+   * Retract request to make the element disabled and restore disabled to previous
+   */
+  public retractRequestToBeDisabled(): void;
+
+  private __internalSetDisabled(value: boolean): void;
+}
+
+export declare function DisabledMixinImplementation<T extends Constructor<HTMLElement>>(
+  superclass: T,
+): T & Constructor<DisabledMixinHost>;
+
+export type DisabledMixin = typeof DisabledMixinImplementation;
diff --git a/packages/core/types/DisabledWithTabIndexMixinTypes.d.ts b/packages/core/types/DisabledWithTabIndexMixinTypes.d.ts
new file mode 100644
index 000000000..358005a3b
--- /dev/null
+++ b/packages/core/types/DisabledWithTabIndexMixinTypes.d.ts
@@ -0,0 +1,29 @@
+import { Constructor } from '@open-wc/dedupe-mixin';
+import { DisabledMixinHost } from './DisabledMixinTypes';
+export declare class DisabledWithTabIndexMixinHost {
+  static get properties(): {
+    tabIndex: {
+      type: NumberConstructor;
+      reflect: boolean;
+      attribute: string;
+    };
+  };
+  tabIndex: number;
+  /**
+   * Makes request to make the element disabled and set the tabindex
+   */
+  public makeRequestToBeDisabled(): void;
+
+  /**
+   * Retract request to make the element disabled and restore disabled and tabindex to previous
+   */
+  public retractRequestToBeDisabled(): void;
+
+  private __internalSetTabIndex(value: boolean): void;
+}
+
+export declare function DisabledWithTabIndexMixinImplementation<T extends Constructor<HTMLElement>>(
+  superclass: T,
+): T & Constructor<DisabledWithTabIndexMixinHost> & Constructor<DisabledMixinHost>;
+
+export type DisabledWithTabIndexMixin = typeof DisabledWithTabIndexMixinImplementation;
diff --git a/packages/core/types/SlotMixinTypes.d.ts b/packages/core/types/SlotMixinTypes.d.ts
new file mode 100644
index 000000000..d911b267f
--- /dev/null
+++ b/packages/core/types/SlotMixinTypes.d.ts
@@ -0,0 +1,53 @@
+import { Constructor } from '@open-wc/dedupe-mixin';
+
+declare function slotFunction(): HTMLElement | undefined;
+
+export type SlotsMap = {
+  [key: string]: typeof slotFunction;
+};
+
+export declare class SlotMixinHost {
+  /**
+   * Obtains all the slots to create
+   */
+  slots: SlotsMap;
+
+  /**
+   * Starts the creation of slots
+   */
+  protected _connectSlotMixin(): void;
+
+  /**
+   * Useful to decide if a given slot should be manipulated depending on if it was auto generated
+   * or not.
+   *
+   * @param {string} slotName Name of the slot
+   * @return {boolean} true if given slot name been created by SlotMixin
+   */
+  protected _isPrivateSlot(slotName: string): boolean;
+}
+
+/**
+ * # SlotMixin
+ *
+ * `SlotMixin`, when attached to the DOM it creates content for defined slots in the Light DOM.
+ * The content element is created using a factory function and is assigned a slot name from the key.
+ * Existing slot content is not overridden.
+ *
+ * The purpose is to have the default content in the Light DOM rather than hidden in Shadow DOM
+ * like default slot content works natively.
+ *
+ * @example
+ * get slots() {
+ *   return {
+ *     ...super.slots,
+ *     // appends <div slot="foo"></div> to the Light DOM of this element
+ *     foo: () => document.createElement('div'),
+ *   };
+ * }
+ */
+export declare function SlotMixinImplementation<T extends Constructor<HTMLElement>>(
+  superclass: T,
+): T & Constructor<SlotMixinHost>;
+
+export type SlotMixin = typeof SlotMixinImplementation;
diff --git a/packages/core/types/UpdateStylesMixinTypes.d.ts b/packages/core/types/UpdateStylesMixinTypes.d.ts
new file mode 100644
index 000000000..a0d931c3c
--- /dev/null
+++ b/packages/core/types/UpdateStylesMixinTypes.d.ts
@@ -0,0 +1,34 @@
+import { Constructor } from '@open-wc/dedupe-mixin';
+
+export type StylesMap = {
+  [key: string]: string;
+};
+export declare class UpdateStylesMixinHost {
+  /**
+   * @example
+   * <my-element>
+   *     <style>
+   *         :host {
+   *          color: var(--foo);
+   *         }
+   *     </style>
+   * </my-element>
+   *
+   * $0.updateStyles({'background': 'orange', '--foo': '#fff'})
+   * Chrome, Firefox: <my-element style="background: orange; --foo: bar;">
+   * IE: <my-element>
+   *     => to head: <style>color: #fff</style>
+   *
+   * @param {StylesMap} updateStyles
+   */
+  public updateStyles(updateStyles: StylesMap): void;
+}
+
+/**
+ * # UpdateStylesMixin
+ */
+declare function UpdateStylesMixinImplementation<T extends Constructor<HTMLElement>>(
+  superclass: T,
+): T & Constructor<UpdateStylesMixinHost>;
+
+export type UpdateStylesMixin = typeof UpdateStylesMixinImplementation;