feat: move declarative html into fast-element (#7482)

# Pull Request

## 📖 Description

Move the declarative HTML runtime from `@microsoft/fast-html` into `@microsoft/fast-element` under a dedicated `@microsoft/fast-element/declarative.js` entrypoint.

This PR:
- moves the declarative runtime, fixtures, scripts, Playwright configs, and package docs into `packages/fast-element`
- updates internal consumers, website/API docs generation, and the WebUI integration workflow to use the new declarative subpath
- removes the `packages/fast-html` workspace and its legacy changefiles

Closes #7314

## 👩‍💻 Reviewer Notes

Review the migration in roughly this order:
1. `packages/fast-element/src/declarative.ts` and `packages/fast-element/package.json` for the new public entrypoint and export wiring.
2. `packages/fast-element/test/declarative/` and `packages/fast-element/scripts/declarative/` for the moved fixture/test infrastructure.
3. `sites/website/scripts/generate-docs.cjs` and `sites/website/src/docs/3.x/` for the new declarative API/docs publication path.

## 📑 Test Plan

- `npm run build`
- `npm run checkchange`
- `npm run test:chromium:declarative -w @microsoft/fast-element`
- `cd packages/fast-element && npx playwright test --config=playwright.declarative.config.ts test/declarative/fixtures/ecosystem/deprecated-event-warning/deprecated-event-warning.spec.ts --workers=1`
- `cd packages/fast-element && npx playwright test --config=playwright.declarative.config.ts test/declarative/fixtures/extensions/attribute-map-naming-strategy/attribute-map-naming-strategy.spec.ts --project=webkit --workers=1`
- `npm run test` in the shared CLI environment reached the declarative Playwright matrix and failed intermittently while another repo was reusing port `5174`; the focused reruns above passed.

## ✅ Checklist

### General

- [x] I have included a change request file using `$ npm run change`
- [ ] I have added tests for my changes.
- [x] I have tested my changes.
- [x] I have updated the project documentation to reflect my changes.
- [x] I have read the [CONTRIBUTING](https://github.com/microsoft/fast/blob/main/CONTRIBUTING.md) documentation and followed the [standards](https://github.com/microsoft/fast/blob/main/CODE_OF_CONDUCT.md#our-standards) for this project.

Author: janechu

.github/copilot-instructions.md

@@ -12,16 +12,15 @@ applyTo: '**/*'
 
 ## Repository Overview
 
-FAST (`@microsoft/fast`) is a Microsoft open-source monorepo for building W3C-standards-compliant Web Components. The core package is `@microsoft/fast-element` — a lightweight, performant Web Component authoring library. Additional packages provide routing and declarative HTML support.
+FAST (`@microsoft/fast`) is a Microsoft open-source monorepo for building W3C-standards-compliant Web Components. The core package is `@microsoft/fast-element` — a lightweight, performant Web Component authoring library with a declarative HTML entrypoint. Additional packages provide routing plus declarative build and test tooling.
 
 ## Packages
 
 Each package includes a DESIGN.md file, read that to gain a general understanding of the code for that package.
 
 | Package | Status | Purpose |
 |---|---|---|
-| `@microsoft/fast-element` | Stable (v2) | Core Web Component library |
-| `@microsoft/fast-html` | Alpha | Declarative HTML parser |
+| `@microsoft/fast-element` | Stable (v2) | Core Web Component library, including declarative HTML via `@microsoft/fast-element/declarative.js` |
 | `@microsoft/fast-router` | Alpha | Web Component router |
 | `@microsoft/fast-build` | Prerelease | Declarative HTML server side renderer |
 | `@microsoft/fast-test-harness` | Prerelease | Testing utilities for FASTElement components |

.github/skills/shipping/SKILL.md

@@ -48,7 +48,7 @@ Use [conventional commit](https://www.conventionalcommits.org/) format for PR ti
 Common types: `feat`, `fix`, `chore`, `docs`, `refactor`, `test`, `perf`.
 
 Examples:
-- `feat: add shadow DOM support to fast-html parser`
+- `feat: add shadow DOM support to declarative templates`
 - `fix: resolve memory leak in template binding`
 - `chore: update dev dependencies`
 

.github/skills/testing/SKILL.md

@@ -27,22 +27,22 @@ All commands are run from the **monorepo root**. Use `-w` to target a specific p
 |---|---|
 | Run all tests (all browsers) | `npm run test` |
 | Run all tests (Chromium only) | `npm run test:chromium` |
-| Test a single package (all browsers) | `npm run test -w @microsoft/fast-html` |
-| Test a single package (Chromium only) | `npm run test:chromium -w @microsoft/fast-html` |
+| Test a single package (all browsers) | `npm run test -w @microsoft/fast-element` |
+| Test a single package (Chromium only) | `npm run test:chromium -w @microsoft/fast-element` |
 | Test changed packages only | `npx lage test:node test:chromium --since origin/main` |
 
 #### Interactive / debug workflows
 
-For `@microsoft/fast-html`, these additional scripts are available:
+For FAST declarative tests in `@microsoft/fast-element`, these additional scripts are available:
 
 | Task | Command |
 |---|---|
-| Playwright UI mode | `npm run test:ui -w @microsoft/fast-html` |
-| Start Vite dev server only | `npm run test-server -w @microsoft/fast-html` |
-| Dev mode (watch + server) | `npm run dev -w @microsoft/fast-html` |
-| Rebuild fixtures | `npm run build:fixtures -w @microsoft/fast-html` |
-| Build fixtures with webui | `npm run build:fixtures:webui -w @microsoft/fast-html` |
-| Run webui integration tests | `npm run test:webui-integration -w @microsoft/fast-html` |
+| Playwright UI mode | `npm run test:ui:declarative -w @microsoft/fast-element` |
+| Start Vite dev server only | `npm run test-server:declarative -w @microsoft/fast-element` |
+| Dev mode (watch + server) | `npm run dev:declarative -w @microsoft/fast-element` |
+| Rebuild fixtures | `npm run build:fixtures -w @microsoft/fast-element` |
+| Build fixtures with webui | `npm run build:fixtures:webui -w @microsoft/fast-element` |
+| Run webui integration tests | `npm run test:webui-integration -w @microsoft/fast-element` |
 
 **Playwright UI mode** (`test:ui`) starts a visual test runner where you can select and debug individual tests, view traces, and inspect DOM snapshots.
 
@@ -80,7 +80,7 @@ A dedicated workflow for validating FAST's integration with `@microsoft/webui`.
 The workflow builds all packages, installs Playwright Chromium, and runs:
 
 ```bash
-npm run test:webui-integration -w @microsoft/fast-html
+npm run test:webui-integration -w @microsoft/fast-element
 ```
 
 This builds each fixture with `webui build --plugin=fast`, renders the protocol with the fixture's `state.json`, and runs the same Playwright specs against the webui-rendered output.
@@ -105,13 +105,13 @@ FAST tests are [Playwright](https://playwright.dev/) integration tests that run
 - Fixture tests live inside their fixture directory: `test/fixtures/<category>/<feature>/<feature>.spec.ts`.
 - Source-level tests are co-located next to the code they test: `src/<feature>/<feature>.pw.spec.ts`.
 
-### Writing fixture tests for fast-html
+### Writing declarative fixture tests
 
-Fixture tests for `@microsoft/fast-html` are the primary way to verify declarative template features. Each fixture is a self-contained test case with its own HTML, state, templates, and component definitions.
+Fixture tests in `@microsoft/fast-element/test/declarative/fixtures` are the primary way to verify declarative template features. Each fixture is a self-contained test case with its own HTML, state, templates, and component definitions.
 
 For a complete guide on creating fixtures — including how to write `entry.html`, `state.json`, `templates.html`, `main.ts`, and spec files — see:
 
-📄 **[Writing Fixtures](../../../packages/fast-html/test/fixtures/WRITING_FIXTURES.md)**
+📄 **[Writing Fixtures](../../../packages/fast-element/test/declarative/fixtures/WRITING_FIXTURES.md)**
 
 ### Quick example
 

.github/workflows/ci-webui-integration.yml

@@ -47,4 +47,4 @@ jobs:
       run: npx playwright install --with-deps chromium
 
     - name: Run WebUI integration tests
-      run: npm run test:webui-integration -w @microsoft/fast-html
+      run: npm run test:webui-integration -w @microsoft/fast-element

change/@microsoft-fast-build-61e3e3bd-e227-4449-9ff6-8edc104819d8.json

@@ -0,0 +1,7 @@
+{
+  "type": "none",
+  "comment": "Update fast-build documentation to reference the fast-element declarative HTML docs.",
+  "packageName": "@microsoft/fast-build",
+  "email": "7559015+janechu@users.noreply.github.com",
+  "dependentChangeType": "none"
+}

change/@microsoft-fast-element-59a62261-1a53-4564-bc64-0c9c1982be71.json

@@ -0,0 +1,7 @@
+{
+  "type": "none",
+  "comment": "Simplify fast-element declarative package documentation and test configuration without changing the published API.",
+  "packageName": "@microsoft/fast-element",
+  "email": "7559015+janechu@users.noreply.github.com",
+  "dependentChangeType": "none"
+}

change/@microsoft-fast-element-move-declarative-html.json

@@ -0,0 +1,7 @@
+{
+  "type": "major",
+  "comment": "Move declarative HTML APIs into @microsoft/fast-element/declarative.js and remove the @microsoft/fast-html package.",
+  "packageName": "@microsoft/fast-element",
+  "dependentChangeType": "none",
+  "email": "7559015+janechu@users.noreply.github.com"
+}