From 580600615b97366962ae4338c2f0db2dfe9cebf3 Mon Sep 17 00:00:00 2001
From: Sean Doyle <sean.p.doyle24@gmail.com>
Date: Wed, 2 Oct 2024 10:37:10 -0400
Subject: [PATCH] Support for `ElementInternals` and Constraint Validations

Integrate with `<form>` elements directly through built-in support for
[ElementInternals][].

This is achieved with graceful degradation in two ways:

* automatically check for browser support
* support globally disabling through `Trix.config.editor.formAssociated = false`

According to the [Form-associated custom elements][] section of [More
capable form controls][], various behaviors that the `<trix-editor>`
element was recreating are provided out of the box.

For example, the `<label>` element support can be achieved through
[ElementInternals.labels][]. Similarly, a `formResetCallback()` will
fire whenever the associated `<form>` element resets.

Add support for integrating with [Constraint validation][] through the
support for the `[required]` attribute and the
`setCustomValidity(message)` method.

[Constraint validation]: https://developer.mozilla.org/en-US/docs/Web/HTML/Constraint_validation
[#1023]: https://github.com/basecamp/trix/issues/1023
[ElementInternals]: https://developer.mozilla.org/en-US/docs/Web/API/ElementInternals
[Form-associated custom elements]: https://web.dev/articles/more-capable-form-controls#form-associated_custom_elements
[More capable form controls]: https://web.dev/articles/more-capable-form-controls
[ElementInternals.setFormValue]: https://developer.mozilla.org/en-US/docs/Web/API/ElementInternals/setFormValue
[ElementInternals.labels]: https://developer.mozilla.org/en-US/docs/Web/API/ElementInternals/labels
---
 .github/workflows/ci.yml                      |   7 +-
 README.md                                     | 132 ++++++++++++++-
 assets/index.html                             |   4 +
 karma.conf.js                                 |  10 +-
 src/test/system/custom_element_test.js        | 160 +++++++++++++++++-
 .../fixtures/editor_with_labels.js            |   8 +-
 .../fixtures/editors_with_forms.js            |   5 +-
 .../fixtures/form_associated_false.js         |   1 +
 src/trix/config/editor.js                     |   3 +
 src/trix/config/index.js                      |   1 +
 src/trix/controllers/editor_controller.js     |   2 +-
 src/trix/elements/trix_editor_element.js      | 157 ++++++++++++++++-
 12 files changed, 473 insertions(+), 17 deletions(-)
 create mode 100644 src/test/test_helpers/fixtures/form_associated_false.js
 create mode 100644 src/trix/config/editor.js

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 8ab86c134..7dd3d3971 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -8,7 +8,12 @@ on:
 
 jobs:
   build:
-    name: Browser tests
+    name: "Browser tests (Trix.config.editor.formAssociated = ${{ matrix.formAssociated }})"
+    strategy:
+      matrix:
+        formAssociated: [true, false]
+    env:
+      FORM_ASSOCIATED: "${{ matrix.formAssociated }}"
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v3
diff --git a/README.md b/README.md
index 2645b79cd..e6f404922 100644
--- a/README.md
+++ b/README.md
@@ -19,7 +19,7 @@ This is the approach that all modern, production ready, WYSIWYG editors now take
 
 <details><summary>Trix supports all evergreen, self-updating desktop and mobile browsers.</summary><img src="https://app.saucelabs.com/browser-matrix/basecamp_trix.svg"></details>
 
-Trix is built with established web standards, notably [Custom Elements](https://developer.mozilla.org/en-US/docs/Web/Web_Components/Using_custom_elements), [Mutation Observer](https://dom.spec.whatwg.org/#mutation-observers), and [Promises](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise).
+Trix is built with established web standards, notably [Custom Elements](https://developer.mozilla.org/en-US/docs/Web/Web_Components/Using_custom_elements), [Element Internals](https://developer.mozilla.org/en-US/docs/Web/API/ElementInternals), [Mutation Observer](https://dom.spec.whatwg.org/#mutation-observers), and [Promises](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise).
 
 # Getting Started
 
@@ -88,6 +88,16 @@ If the attribute is defined in `Trix.config.blockAttributes`, Trix will apply th
 
 Clicking the quote button toggles whether the block should be rendered with `<blockquote>`.
 
+## Integrating with Element Internals
+
+Trix will integrate `<trix-editor>` elements with forms depending on the browser's support for [Element Internals](https://developer.mozilla.org/en-US/docs/Web/API/ElementInternals). By default, Trix will enable support for `ElementInternals` when the feature is enabled in the browser. If there is a need to disable support for `ElementInternals`, set `Trix.config.editor.formAssociated = false`:
+
+```js
+import Trix from "trix"
+
+Trix.config.editor.formAssociated = false
+```
+
 ## Invoking Internal Trix Actions
 
 Internal actions are defined in `controllers/editor_controller.js` and consist of:
@@ -148,6 +158,126 @@ To populate a `<trix-editor>` with stored content, include that content in the a
 
 Always use an associated input element to safely populate an editor. Trix won’t load any HTML content inside a `<trix-editor>…</trix-editor>` tag.
 
+## Validating the Editor
+
+Out of the box, `<trix-editor>` elements support browsers' built-in [Constraint
+validation][]. When rendered with the [required][] attribute, editors will be
+invalid when they're completely empty. For example, consider the following HTML:
+
+```html
+<input id="x" value="" type="hidden" name="content">
+<trix-editor input="x" required></trix-editor>
+```
+
+Since the `<trix-editor>` element is `[required]`, it is invalid when its value
+is empty:
+
+```js
+const editor = document.querySelector("trix-editor")
+
+editor.validity.valid        // => false
+editor.validity.valueMissing // => true
+editor.matches(":valid")     // => false
+editor.matches(":invalid")   // => true
+
+editor.value = "A value that isn't empty"
+
+editor.validity.valid         // => true
+editor.validity.valueMissing  // => false
+editor.matches(":valid")      // => true
+editor.matches(":invalid")    // => false
+```
+
+In addition to the built-in `[required]` attribute, `<trix-editor>`
+elements support custom validation through their [setCustomValidity][] method.
+For example, consider the following HTML:
+
+```js
+<input id="x" value="" type="hidden" name="content">
+<trix-editor input="x"></trix-editor>
+```
+
+Custom validation can occur at any time. For example, validation can occur after
+a `trix-change` event fired after the editor's contents change:
+
+```js
+addEventListener("trix-change", (event) => {
+  const editorElement = event.target
+  const trixDocument = editorElement.editor.getDocument()
+  const isValid = (trixDocument) => {
+    // determine the validity based on your custom criteria
+    return true
+  }
+
+  if (isValid(trixDocument)) {
+    editorElement.setCustomValidity("")
+  } else {
+    editorElement.setCustomValidity("The document is not valid.")
+  }
+}
+```
+
+[Constraint validation]: https://developer.mozilla.org/en-US/docs/Web/HTML/Constraint_validation
+[required]: https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes/required
+[setCustomValidity]: https://developer.mozilla.org/en-US/docs/Web/API/HTMLObjectElement/setCustomValidity
+
+## Disabling the Editor
+
+To disable the `<trix-editor>`, render it with the `[disabled]` attribute:
+
+```html
+<trix-editor disabled></trix-editor>
+```
+
+Disabled editors are not editable, cannot receive focus, and their values will
+be ignored when their related `<form>` element is submitted.
+
+To change whether or not an editor is disabled, either toggle the `[disabled]`
+attribute or assign a boolean to the `.disabled` property:
+
+```html
+<trix-editor id="editor" disabled></trix-editor>
+
+<script>
+  const editor = document.getElementById("editor")
+
+  editor.toggleAttribute("disabled", false)
+  editor.disabled = true
+</script>
+```
+
+When disabled, the editor will match the [:disabled CSS
+pseudo-class][:disabled].
+
+[:disabled]: https://developer.mozilla.org/en-US/docs/Web/CSS/:disabled
+
+## Providing an Accessible Name
+
+Like other form controls, `<trix-editor>` elements should have an accessible name. The `<trix-editor>` element integrates with `<label>` elements and The `<trix-editor>` supports two styles of integrating with `<label>` elements:
+
+1. render the `<trix-editor>` element with an `[id]` attribute that the `<label>` element references through its `[for]` attribute:
+
+```html
+<label for="editor">Editor</label>
+<trix-editor id="editor"></trix-editor>
+```
+
+2. render the `<trix-editor>` element as a child of the `<label>` element:
+
+```html
+<trix-toolbar id="editor-toolbar"></trix-toolbar>
+<label>
+  Editor
+
+  <trix-editor toolbar="editor-toolbar"></trix-editor>
+</label>
+```
+
+> [!WARNING]
+> When rendering the `<trix-editor>` element as a child of the `<label>` element, [explicitly render](#creating-an-editor) the corresponding `<trix-toolbar>` element outside of the `<label>` element.
+
+In addition to integrating with `<label>` elements, `<trix-editor>` elements support `[aria-label]` and `[aria-labelledby]` attributes.
+
 ## Styling Formatted Content
 
 To ensure what you see when you edit is what you see when you save, use a CSS class name to scope styles for Trix formatted content. Apply this class name to your `<trix-editor>` element, and to a containing element when you render stored Trix content for display in your application.
diff --git a/assets/index.html b/assets/index.html
index e4cf73b85..65e97b39b 100644
--- a/assets/index.html
+++ b/assets/index.html
@@ -17,6 +17,10 @@
         max-width: 700px;
       }
 
+      trix-editor:invalid {
+        border: solid 1px red;
+      }
+
       #output {
         margin: 1rem 0 0;
       }
diff --git a/karma.conf.js b/karma.conf.js
index 9fe317624..fc5c6ad82 100644
--- a/karma.conf.js
+++ b/karma.conf.js
@@ -3,7 +3,7 @@ const config = {
   frameworks: [ "qunit" ],
   files: [
     { pattern: "dist/test.js", watched: false },
-    { pattern: "src/test_helpers/fixtures/*.png", watched: false, included: false, served: true }
+    { pattern: "src/test/test_helpers/fixtures/*.png", watched: false, included: false, served: true }
   ],
   proxies: {
     "/test_helpers/fixtures/": "/base/src/test_helpers/fixtures/"
@@ -29,6 +29,14 @@ const config = {
 
 /* eslint camelcase: "off",  */
 
+if (process.env.FORM_ASSOCIATED === "false") {
+  config.files.push({
+    pattern: "src/test/test_helpers/fixtures/form_associated_false.js",
+    watched: false,
+    included: true
+  })
+}
+
 if (process.env.SAUCE_ACCESS_KEY) {
   config.customLaunchers = {
     sl_chrome_latest: {
diff --git a/src/test/system/custom_element_test.js b/src/test/system/custom_element_test.js
index 00b2f2250..5af7d6043 100644
--- a/src/test/system/custom_element_test.js
+++ b/src/test/system/custom_element_test.js
@@ -1,3 +1,4 @@
+import * as config from "trix/config"
 import { rangesAreEqual } from "trix/core/helpers"
 
 import {
@@ -500,7 +501,7 @@ testGroup("Custom element API", { template: "editor_empty" }, () => {
 testGroup("<label> support", { template: "editor_with_labels" }, () => {
   test("associates all label elements", () => {
     const labels = [ document.getElementById("label-1"), document.getElementById("label-3") ]
-    assert.deepEqual(getEditorElement().labels, labels)
+    assert.deepEqual(Array.from(getEditorElement().labels), labels)
   })
 
   test("focuses when <label> clicked", () => {
@@ -538,4 +539,161 @@ testGroup("form property references its <form>", { template: "editors_with_forms
     const editor = document.getElementById("editor-with-no-form")
     assert.equal(editor.form, null)
   })
+
+  test("editor resets to its original value on element reset", async () => {
+    const element = getEditorElement()
+
+    await typeCharacters("hello")
+    element.reset()
+    expectDocument("\n")
+  })
+
+  test("element returns empty string when value is missing", () => {
+    const element = getEditorElement()
+
+    assert.equal(element.value, "")
+  })
+
+  test("editor returns its type", () => {
+    const element = getEditorElement()
+
+    assert.equal("trix-editor", element.type)
+  })
+
+  testIfFormAssociated("adds [disabled] attribute based on .disabled property", () => {
+    const editor = document.getElementById("editor-with-ancestor-form")
+
+    editor.disabled = true
+
+    assert.equal(editor.hasAttribute("disabled"), true, "adds [disabled] attribute")
+
+    editor.disabled = false
+
+    assert.equal(editor.hasAttribute("disabled"), false, "removes [disabled] attribute")
+  })
+
+  testIfFormAssociated("removes [contenteditable] and disables input when editor element has [disabled]", () => {
+    const editor = document.getElementById("editor-with-no-form")
+
+    editor.setAttribute("disabled", "")
+
+    assert.equal(editor.matches(":disabled"), true, "sets :disabled CSS pseudostate")
+    assert.equal(editor.inputElement.disabled, true, "disables input")
+    assert.equal(editor.disabled, true, "exposes [disabled] attribute as .disabled property")
+    assert.equal(editor.hasAttribute("contenteditable"), false, "removes [contenteditable] attribute")
+
+    editor.removeAttribute("disabled")
+
+    assert.equal(editor.matches(":disabled"), false, "removes sets :disabled pseudostate")
+    assert.equal(editor.inputElement.disabled, false, "enabled input")
+    assert.equal(editor.disabled, false, "updates .disabled property")
+    assert.equal(editor.hasAttribute("contenteditable"), true, "adds [contenteditable] attribute")
+  })
+
+  testIfFormAssociated("removes [contenteditable] and disables input when editor element is :disabled", () => {
+    const editor = document.getElementById("editor-within-fieldset")
+    const fieldset = document.getElementById("fieldset")
+
+    fieldset.disabled = true
+
+    assert.equal(editor.matches(":disabled"), true, "sets :disabled CSS pseudostate")
+    assert.equal(editor.inputElement.disabled, true, "disables input")
+    assert.equal(editor.disabled, true, "infers disabled state from ancestor")
+    assert.equal(editor.hasAttribute("disabled"), false, "does not set [disabled] attribute")
+    assert.equal(editor.hasAttribute("contenteditable"), false, "removes [contenteditable] attribute")
+
+    fieldset.disabled = false
+
+    assert.equal(editor.matches(":disabled"), false, "removes sets :disabled pseudostate")
+    assert.equal(editor.inputElement.disabled, false, "enabled input")
+    assert.equal(editor.disabled, false, "updates .disabled property")
+    assert.equal(editor.hasAttribute("disabled"), false, "does not set [disabled] attribute")
+    assert.equal(editor.hasAttribute("contenteditable"), true, "adds [contenteditable] attribute")
+  })
+
+  testIfFormAssociated("does not receive focus when :disabled", () => {
+    const activeEditor = document.getElementById("editor-with-input-form")
+    const editor = document.getElementById("editor-within-fieldset")
+
+    activeEditor.focus()
+    editor.disabled = true
+    editor.focus()
+
+    assert.equal(activeEditor, document.activeElement, "disabled editor does not receive focus")
+  })
+
+  testIfFormAssociated("disabled editor does not encode its value when the form is submitted", () => {
+    const editor = document.getElementById("editor-with-ancestor-form")
+    const form = editor.form
+
+    editor.inputElement.value = "Hello world"
+    editor.disabled = true
+
+    assert.deepEqual({}, Object.fromEntries(new FormData(form).entries()), "does not write to FormData")
+  })
+
+  testIfFormAssociated("validates with [required] attribute as invalid", () => {
+    const editor = document.getElementById("editor-with-ancestor-form")
+    const form = editor.form
+    let invalidEvent, submitEvent = null
+
+    editor.addEventListener("invalid", event => invalidEvent = event, { once: true })
+    form.addEventListener("submit", event => submitEvent = event, { once: true })
+
+    editor.required = true
+    form.requestSubmit()
+
+    // assert.equal(document.activeElement, editor, "editor receives focus")
+    assert.equal(editor.required, true, ".required property retrurns true")
+    assert.equal(editor.validity.valid, false, "validity.valid is false")
+    assert.equal(editor.validationMessage, "Please fill out this field.", "sets .validationMessage")
+    assert.equal(invalidEvent.target, editor, "dispatches 'invalid' event on editor")
+    assert.equal(submitEvent, null, "does not dispatch a 'submit' event")
+  })
+
+  testIfFormAssociated("does not validate with [disabled] attribute", () => {
+    const editor = document.getElementById("editor-with-ancestor-form")
+    let invalidEvent = null
+
+    editor.disabled = true
+    editor.required = true
+    editor.addEventListener("invalid", event => invalidEvent = event, { once: true })
+    editor.reportValidity()
+
+    assert.equal(invalidEvent, null, "does not dispatch an 'invalid' event")
+  })
+
+  testIfFormAssociated("re-validates when the value changes", async () => {
+    const editor = document.getElementById("editor-with-ancestor-form")
+    editor.required = true
+    editor.focus()
+
+    assert.equal(editor.validity.valid, false, "validity.valid is initially false")
+
+    await typeCharacters("a")
+
+    assert.equal(editor.validity.valid, true, "validity.valid is true after re-validating")
+    assert.equal(editor.validity.valueMissing, false, "validity.valueMissing is false")
+    assert.equal(editor.validationMessage, "", "clears the validationMessage")
+  })
+
+  testIfFormAssociated("accepts a customError validation message", () => {
+    const editor = document.getElementById("editor-with-ancestor-form")
+
+    editor.setCustomValidity("A custom validation message")
+
+    assert.equal(editor.validity.valid, false)
+    assert.equal(editor.validity.customError, true)
+    assert.equal(editor.validationMessage, "A custom validation message")
+  })
 })
+
+function testIfFormAssociated(name, callback) {
+  test(name, async () => {
+    if (config.editor.formAssociated) {
+      await callback()
+    } else {
+      assert.equal(config.editor.formAssociated, false, "skipping test that requires ElementInternals")
+    }
+  })
+}
diff --git a/src/test/test_helpers/fixtures/editor_with_labels.js b/src/test/test_helpers/fixtures/editor_with_labels.js
index e1a1fa0a0..8d174e124 100644
--- a/src/test/test_helpers/fixtures/editor_with_labels.js
+++ b/src/test/test_helpers/fixtures/editor_with_labels.js
@@ -1,7 +1,5 @@
 export default () =>
   `<label id="label-1" for="editor"><span>Label 1</span></label>
-  <label id="label-2">
-    Label 2
-    <trix-editor id="editor"></trix-editor>
-  </label>
-  <label id="label-3" for="editor">Label 3</label>`
+   <label id="label-2">Label 2</label>
+   <trix-editor id="editor"></trix-editor>
+   <label id="label-3" for="editor">Label 3</label>`
diff --git a/src/test/test_helpers/fixtures/editors_with_forms.js b/src/test/test_helpers/fixtures/editors_with_forms.js
index 56ed494ac..292d0e315 100644
--- a/src/test/test_helpers/fixtures/editors_with_forms.js
+++ b/src/test/test_helpers/fixtures/editors_with_forms.js
@@ -1,6 +1,6 @@
 export default () =>
   `<form id="ancestor-form">
-    <trix-editor id="editor-with-ancestor-form"></trix-editor>
+    <trix-editor id="editor-with-ancestor-form" name="editor-with-ancestor-form"></trix-editor>
   </form>
 
   <form id="input-form">
@@ -8,4 +8,5 @@ export default () =>
   </form>
   <trix-editor id="editor-with-input-form" input="hidden-input"></trix-editor>
 
-  <trix-editor id="editor-with-no-form"></trix-editor>`
+  <trix-editor id="editor-with-no-form"></trix-editor>
+  <fieldset id="fieldset"><trix-editor id="editor-within-fieldset"></fieldset>`
diff --git a/src/test/test_helpers/fixtures/form_associated_false.js b/src/test/test_helpers/fixtures/form_associated_false.js
new file mode 100644
index 000000000..2b95f2bc5
--- /dev/null
+++ b/src/test/test_helpers/fixtures/form_associated_false.js
@@ -0,0 +1 @@
+window.Trix.config.editor.formAssociated = false
diff --git a/src/trix/config/editor.js b/src/trix/config/editor.js
new file mode 100644
index 000000000..cc11f166f
--- /dev/null
+++ b/src/trix/config/editor.js
@@ -0,0 +1,3 @@
+export default {
+  formAssociated: "ElementInternals" in window
+}
diff --git a/src/trix/config/index.js b/src/trix/config/index.js
index 27f7abc2f..64684bc1e 100644
--- a/src/trix/config/index.js
+++ b/src/trix/config/index.js
@@ -2,6 +2,7 @@ export { default as attachments } from "./attachments"
 export { default as blockAttributes } from "./block_attributes"
 export { default as browser } from "./browser"
 export { default as css } from "./css"
+export { default as editor } from "./editor"
 export { default as fileSize } from "./file_size_formatting"
 export { default as input } from "./input"
 export { default as keyNames } from "./key_names"
diff --git a/src/trix/controllers/editor_controller.js b/src/trix/controllers/editor_controller.js
index 6f469db5a..04c2e921d 100644
--- a/src/trix/controllers/editor_controller.js
+++ b/src/trix/controllers/editor_controller.js
@@ -503,7 +503,7 @@ export default class EditorController extends Controller {
   updateInputElement() {
     const element = this.compositionController.getSerializableElement()
     const value = serializeToContentType(element, "text/html")
-    return this.editorElement.setInputElementValue(value)
+    return this.editorElement.setFormValue(value)
   }
 
   notifyEditorElement(message, data) {
diff --git a/src/trix/elements/trix_editor_element.js b/src/trix/elements/trix_editor_element.js
index 44626100b..e99a898e5 100644
--- a/src/trix/elements/trix_editor_element.js
+++ b/src/trix/elements/trix_editor_element.js
@@ -161,6 +161,19 @@ installDefaultCSSForTagName("trix-editor", `\
 }`)
 
 export default class TrixEditorElement extends HTMLElement {
+  static get formAssociated() {
+    return config.editor.formAssociated
+  }
+
+  #customValidationMessage
+  #internals
+
+  constructor() {
+    super()
+    this.#internals = this.constructor.formAssociated ?
+      this.attachInternals() :
+      null
+  }
 
   // Properties
 
@@ -174,6 +187,8 @@ export default class TrixEditorElement extends HTMLElement {
   }
 
   get labels() {
+    if (this.#internals) return this.#internals.labels
+
     const labels = []
     if (this.id && this.ownerDocument) {
       labels.push(...Array.from(this.ownerDocument.querySelectorAll(`label[for='${this.id}']`) || []))
@@ -238,6 +253,76 @@ export default class TrixEditorElement extends HTMLElement {
     this.editor?.loadHTML(this.defaultValue)
   }
 
+  get type() {
+    return this.localName
+  }
+
+  get disabled() {
+    if (this.#internals) {
+      return this.inputElement.disabled
+    } else {
+      console.warn("Trix is not configured to support the [disabled] attribute. Set Trix.config.editor.formAssociated = true")
+
+      return false
+    }
+  }
+
+  set disabled(value) {
+    if (this.#internals) {
+      this.toggleAttribute("disabled", value)
+    } else {
+      console.warn("Trix is not configured to support the [disabled] attribute. Set Trix.config.editor.formAssociated = true")
+    }
+  }
+
+  get required() {
+    if (this.#internals) {
+      return this.hasAttribute("required")
+    } else {
+      console.warn("Trix is not configured to support the [required] attribute. Set Trix.config.editor.formAssociated = true")
+
+      return false
+    }
+  }
+
+  set required(value) {
+    if (this.#internals) {
+      this.toggleAttribute("required", value)
+      this.#synchronizeValidation()
+    } else {
+      console.warn("Trix is not configured to support the [required] attribute. Set Trix.config.editor.formAssociated = true")
+    }
+  }
+
+  get validity() {
+    if (this.#internals) {
+      return this.#internals.validity
+    } else {
+      console.warn("Trix is not configured to support the validity property. Set Trix.config.editor.formAssociated = true")
+      return null
+    }
+  }
+
+  get validationMessage() {
+    if (this.#internals) {
+      return this.#internals.validationMessage
+    } else {
+      console.warn("Trix is not configured to support the validationMessage property. Set Trix.config.editor.formAssociated = true")
+
+      return ""
+    }
+  }
+
+  get willValidate() {
+    if (this.#internals) {
+      return this.#internals.willValidate
+    } else {
+      console.warn("Trix is not configured to support the willValidate property. Set Trix.config.editor.formAssociated = true")
+
+      return false
+    }
+  }
+
   // Controller delegate methods
 
   notify(message, data) {
@@ -246,10 +331,14 @@ export default class TrixEditorElement extends HTMLElement {
     }
   }
 
-  setInputElementValue(value) {
+  setFormValue(value) {
     if (this.inputElement) {
       this.inputElement.value = value
     }
+
+    if (this.#internals) {
+      this.#synchronizeValidation()
+    }
   }
 
   // Element lifecycle
@@ -269,16 +358,25 @@ export default class TrixEditorElement extends HTMLElement {
         requestAnimationFrame(() => triggerEvent("trix-initialize", { onElement: this }))
       }
       this.editorController.registerSelectionManager()
-      this.registerResetListener()
-      this.registerClickListener()
+      if (this.#internals) {
+        this.#synchronizeValidation()
+      } else {
+        this.registerResetListener()
+        this.registerClickListener()
+      }
       autofocus(this)
     }
   }
 
   disconnectedCallback() {
     this.editorController?.unregisterSelectionManager()
-    this.unregisterResetListener()
-    return this.unregisterClickListener()
+    if (this.#internals) {
+      // no-op
+    } else {
+      this.unregisterResetListener()
+      return this.unregisterClickListener()
+    }
+
   }
 
   // Form support
@@ -322,4 +420,53 @@ export default class TrixEditorElement extends HTMLElement {
   reset() {
     this.value = this.defaultValue
   }
+
+  checkValidity() {
+    if (this.#internals) {
+      return this.#internals.checkValidity()
+    } else {
+      console.warn("Trix is not configured to support checkValidity(). Set Trix.config.editor.formAssociated = true")
+
+      return true
+    }
+  }
+
+  reportValidity() {
+    if (this.#internals) {
+      return this.#internals.reportValidity()
+    } else {
+      console.warn("Trix is not configured to support reportValidity(). Set Trix.config.editor.formAssociated = true")
+
+      return true
+    }
+  }
+
+  setCustomValidity(validationMessage) {
+    if (this.#internals) {
+      this.#customValidationMessage = validationMessage
+
+      this.#synchronizeValidation()
+    } else {
+      console.warn("Trix is not configured to support setCustomValidity(validationMessage). Set Trix.config.editor.formAssociated = true")
+    }
+  }
+
+  formDisabledCallback(disabled) {
+    this.inputElement.disabled = disabled
+    this.toggleAttribute("contenteditable", !disabled)
+  }
+
+  formResetCallback() {
+    this.reset()
+  }
+
+  #synchronizeValidation() {
+    const { required, value } = this
+    const valueMissing = required && !value
+    const customError = !!this.#customValidationMessage
+    const input = Object.assign(document.createElement("input"), { required })
+    const validationMessage = this.#customValidationMessage || input.validationMessage
+
+    this.#internals.setValidity({ valueMissing, customError }, validationMessage)
+  }
 }