chore: create-a-form docs

docs/guides/how-to/create-a-custom-field.md

@@ -41,7 +41,7 @@ class DummySlider extends LitElement {
         height: 24px;
         width: 24px;
         top: 50%;
-        transform: translateY(-50%);
+        transform: translateY(-50%) translateX(-50%);
         transition: left 0.5s ease 0s;
       }
     `,

docs/guides/how-to/create-a-form.md

@@ -0,0 +1,88 @@
+# How To >> Create a form ||15
+
+Creating your own forms as an Application Developer is really simple. Only two rules need to be followed.
+
+This guide is written for Application Developers that work with a fully styled and configured extension of Lion.
+Say for instance we have a 'Wolf' extension layer with a set of styled/configured Lion form components:
+
+- wolf-form
+- wolf-fieldset
+- wolf-input
+- wolf-textarea
+- ...and all other form components
+
+## Two simple rules
+
+When building a form, keep these things in mind:
+
+- avoid shadow roots inside your form
+- do not extend form components
+
+## Avoiding shadow roots
+
+Large or complex forms might need to be split up into multiple files. This composition could be achieved via:
+
+- shared templates
+- (web) components
+
+In the latter case: make sure that your components don't have a shadow root.
+This can be achieved in LitElement by providing the host as render root:
+
+```js
+class MySubForm extends LitElement {
+  // This will make sure the component does not have a shadow root
+  createRenderRoot() {
+    return this;
+  }
+
+  render() {
+    return html`
+      <wolf-fieldset name="my-fieldset">
+        <wolf-input name="my-input"></wolf-input>
+        <wolf-texarea name="my-texarea"></wolf-texarea>
+      </wolf-fieldset>
+    `;
+  }
+}
+customElements.define('my-sub-form', MySubForm);
+```
+
+Our component can be used in our form like this:
+
+```js
+class MyForm extends LitElement {
+  render() {
+    return html`
+      <wolf-form>
+        <form>
+          <my-sub-form></my-sub-form>
+        </form>
+      </wolf-form>
+    `;
+  }
+}
+```
+
+> Today, rendering to light dom is the only way to [stay accessible](../../fundamentals/rationales/accessibility.md#shadow-roots-and-accessibility).
+
+The registration mechanism relies on light dom by design: as soon as you do start to use shadow roots, you will notice that your form components won't register themselves anymore to their parents.
+
+## Extending form components
+
+Extending form components is not recommended for Application Developers: it should be left to the team that creates a Design System (Subclassers).
+This team will add the styling and default configuration needed to make it fit the particular Design System.
+
+With the toolbox provided by the team that creates your Design System, you should be able to build all your forms. Only if you have a really good reason, look into
+[exceptional cases](#exceptional-cases) below.
+
+### Exceptional cases
+
+If the form component you need is still missing, you have two options:
+
+- is it a generic primitive that should be part of Lion? Request it in [Lion discussions](https://github.com/ing-bank/lion/discussions)
+- is it a non generic component that is unique to your app?
+  - follow [create a custom field](./create-a-custom-field.md)
+  - follow [extend a native input](./extend-a-native-input.md)
+
+It's also possible to extend other components, like LionListbox or LionCombobox. You need to be aware that you're building on top of protected apis.
+Changing/overriding functionality is only recommended if the source code in `form-core` is fully understood.

docs/guides/how-to/extend-a-native-input.md

@@ -1,4 +1,4 @@
-# How To >> Extend a native Input ||20
+# How To >> Extend a native input ||20
 
 ```js script
 import { html } from '@mdjs/mdjs-preview';