diff --git a/README.md b/README.md
index dcd079c..607052c 100644
--- a/README.md
+++ b/README.md
@@ -36,6 +36,15 @@ There are many ways to get in touch:
 1. [fast](https://github.com/microsoft/fast) - When I found that Microsoft has their own base class I thought it was super cool!
 1. [lit](https://github.com/lit/lit) - `lit-html` continues to amaze me and I worked to make `wcb` generic so I (and others) can continue to use it
 
+## Size change log
+
+A running record of how each correctness/feature change affects the `WebComponent` base class bundle, measured as min + brotli via [size-limit](https://github.com/ai/size-limit). Baseline before the Phase 0 correctness work: **1.19 kB** (limit 1.2 kB).
+
+| Change                                                           | Size (min+brotli)  | Limit         | Reason & benefit                                                                                                                                                                                                                                                                                                                                                 |
+| ---------------------------------------------------------------- | ------------------ | ------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
+| Move default-attribute reflection out of the constructor         | ~1.19 kB (≈0)      | 1.2 kB        | **Custom Elements Spec compliance.** Constructors may not mutate attributes, so `document.createElement` on a class with `static props` threw `NotSupportedError` in real browsers. Defaults now reflect on first connect, and markup/SSR-provided attributes win. Pure code movement — no net size cost.                                                        |
+| Safe prop cloning (functions/instances) + define-time validation | 1.31 kB (+0.12 kB) | 1.2 → 1.35 kB | **No more `DataCloneError`.** `structuredClone` of a function/class-instance default crashed construction. Defaults are now cloned per key (plain data deep-copied so instances don't share object/array state; non-cloneable values kept by reference), and a one-time warning names any default whose type can't reflect to an attribute. Limit raised to fit. |
+
 ---
 
 _Just keep building._<br>
diff --git a/docs/src/content/docs/guides/library-size.md b/docs/src/content/docs/guides/library-size.md
index ae22065..aa03807 100644
--- a/docs/src/content/docs/guides/library-size.md
+++ b/docs/src/content/docs/guides/library-size.md
@@ -5,4 +5,4 @@ slug: library-size
 
 All the functions and the base class in the library are minimalist by design and only contains what is needed for their purpose.
 
-The main export (with `WebComponent` + `html`) is **1.7 kB** (min + gzip) according to [bundlephobia.com](https://bundlephobia.com/package/web-component-base@latest), and the `WebComponent` base class is **1.08 kB** (min + brotli) according to [size-limit](http://github.com/ai/size-limit).
+The main export (with `WebComponent` + `html`) is **1.7 kB** (min + gzip) according to [bundlephobia.com](https://bundlephobia.com/package/web-component-base@latest), and the `WebComponent` base class is **1.31 kB** (min + brotli) according to [size-limit](http://github.com/ai/size-limit).
diff --git a/package.json b/package.json
index 1836611..8e10359 100644
--- a/package.json
+++ b/package.json
@@ -81,7 +81,7 @@
   "size-limit": [
     {
       "path": "./dist/WebComponent.js",
-      "limit": "1.2 KB"
+      "limit": "1.35 KB"
     },
     {
       "path": "./dist/html.js",
diff --git a/src/WebComponent.js b/src/WebComponent.js
index a9e64d9..88bdc39 100644
--- a/src/WebComponent.js
+++ b/src/WebComponent.js
@@ -11,6 +11,39 @@ import {
   deserialize,
 } from './utils/index.js'
 
+/** Component classes whose defaults have been validated (once per class). */
+const validated = new WeakSet()
+
+/**
+ * Returns a fresh defaults object for a component: plain data is deep-copied
+ * per key so instances never share object/array defaults, while non-cloneable
+ * values (functions, class instances) are kept by reference instead of
+ * throwing `DataCloneError`. On the first call per class it also warns (once)
+ * about types that can't reflect to an attribute — those belong in
+ * handlers/refs, not reflected props.
+ * @param {typeof WebComponent} ctor the component constructor
+ * @returns {object} a fresh defaults object
+ */
+function cloneDefaults(ctor) {
+  const check = !validated.has(ctor)
+  if (check) validated.add(ctor)
+  const out = {}
+  for (const key in ctor.props) {
+    const value = ctor.props[key]
+    const type = typeof value
+    if (check && (type === 'function' || type === 'symbol'))
+      console.warn(
+        `${ctor.name}.${key}: ${type} default not reflectable; use handlers/refs.`
+      )
+    try {
+      out[key] = structuredClone(value)
+    } catch {
+      out[key] = value
+    }
+  }
+  return out
+}
+
 /**
  * A minimal base class to reduce the complexity of creating reactive custom elements
  * @see https://webcomponent.io
@@ -158,7 +191,7 @@ export class WebComponent extends HTMLElement {
   }
 
   #initializeProps() {
-    let initialProps = structuredClone(this.constructor.props) ?? {}
+    let initialProps = cloneDefaults(this.constructor)
     Object.keys(initialProps).forEach((camelCase) => {
       this.#typeMap[camelCase] = typeof initialProps[camelCase]
     })
diff --git a/test/WebComponent.test.mjs b/test/WebComponent.test.mjs
index 70d8ff5..c6e2260 100644
--- a/test/WebComponent.test.mjs
+++ b/test/WebComponent.test.mjs
@@ -325,3 +325,44 @@ describe('default reflection (no setAttribute in constructor)', () => {
     expect(el.getAttribute('my-name')).toBe('Ayo')
   })
 })
+
+/**
+ * Non-cloneable defaults (functions, class instances) must not crash the
+ * constructor via structuredClone's DataCloneError; they are kept by
+ * reference and flagged once with a readable warning.
+ */
+describe('safe prop cloning + define-time validation', () => {
+  it('constructs with a function default (kept by ref) and warns once per class', () => {
+    const warn = vi.spyOn(console, 'warn').mockImplementation(() => {})
+    class WithFn extends WebComponent {
+      static props = { fn: () => 42 }
+    }
+    const tag = 'clone-with-fn'
+    window.customElements.define(tag, WithFn)
+
+    const a = document.createElement(tag)
+    const b = document.createElement(tag)
+    // function survives by reference and is callable on both instances
+    expect(a.props.fn()).toBe(42)
+    expect(b.props.fn()).toBe(42)
+    // memoized per class: constructing many instances warns at most once
+    expect(warn).toHaveBeenCalledOnce()
+    expect(warn.mock.calls[0][0]).toContain('WithFn.fn')
+
+    warn.mockRestore()
+  })
+
+  it('deep-copies object/array defaults so instances do not share them', () => {
+    class WithList extends WebComponent {
+      static props = { list: [1, 2] }
+    }
+    const tag = 'clone-with-list'
+    window.customElements.define(tag, WithList)
+
+    const a = document.createElement(tag)
+    const b = document.createElement(tag)
+    a.props.list.push(3)
+    expect(a.props.list).toEqual([1, 2, 3])
+    expect(b.props.list).toEqual([1, 2])
+  })
+})