Add streaming HTML insertion methods to Element, ShadowRoot, and ChildNode

[noamr]

Add streaming HTML insertion methods to Element, ShadowRoot, and ChildNode

Introduce safe and unsafe streaming HTML parsing and insertion methods:

• Element & ShadowRoot: streamHTML(), streamHTMLUnsafe(), streamAppendHTML(), streamAppendHTMLUnsafe(), streamPrependHTML(), and streamPrependHTMLUnsafe()
• ChildNode: streamBeforeHTML(), streamBeforeHTMLUnsafe(), streamAfterHTML(), streamAfterHTMLUnsafe(), streamReplaceWithHTML(), and streamReplaceWithHTMLUnsafe()

This allows developers to parse and stream HTML directly into specific positions of the DOM tree using WritableStream.

• Stream Semantics:
  • writeAlgorithm: converts chunks to DOMString, revalidates that referenceChild (if present) is still a child of target (aborting the parser and rejecting with HierarchyRequestError if displaced), feeds the chunk to the parser, and resolves when characters and parser-blocking scripts have executed.
  • closeAlgorithm: revalidates referenceChild, appends an explicit EOF to the parser's input stream to finish parsing, and executes deferred and module scripts.
  • abortAlgorithm: aborts the underlying HTML parser.
• Script Execution Model:
  Streaming fragment parsers can execute scripts when runScripts is true:
  • The parser is given a script target document. Script elements created by the parser receive an intended document set to this document.
  • For safe streams or when runScripts is false (default), the script target document is the temporary inert document, preventing script execution.
  • When runScripts is true, the script target document is the context element's node document.
  • Safe streaming into a <script> element throws a NotSupportedError DOMException.
• Atomic Validation & DOM Mutation Ordering:
  • HTMLTemplateElement insertion targets are redirected to their content DocumentFragment before any mutation.
  • For ChildNode methods, disconnected nodes are validated via parent for HTML streaming (throwing HierarchyRequestError if no suitable parent exists).
  • Synchronous option validation (Trusted Types and SanitizerConfig) occurs before the stream is returned and prior to any destructive DOM mutations (replace all with null for streamHTML or removal of the node for streamReplaceWithHTML).
• Trusted Types Integration:
  Unsafe streaming methods accept (SetHTMLUnsafeOptions or TrustedHTMLParserOptions) using the Stream sink type.

Closes #2142

• At least two implementers are interested (and none opposed):
  • Chrome
  • Gecko (HTML streaming & revamped DOM parsing mozilla/standards-positions#1370)
• Tests are written and can be reviewed and commented upon at:
  • Un-tentative streamHTML tests web-platform-tests/wpt#61748
• Implementation bugs are filed:
  • Chromium: Implemented
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=315741
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=2043150
• Corresponding HTML AAM & ARIA in HTML issues & PRs: N/A
• MDN issue is filed: Will be done as part of Blink shipping
• The top of this comment includes a [clear commit message

(https://github.com/whatwg/meta/blob/main/COMMITTING.md) to use.

(See WHATWG Working Mode: Changes for more details.)

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/dynamic-markup-insertion.html ( diff )
/index.html ( diff )
/infrastructure.html ( diff )
/parsing.html ( diff )
/references.html ( diff )
/scripting.html ( diff )
/xhtml.html ( diff )

[zcorpan]

LGTM % spaces

[zcorpan]

I squashed this and rebased it on #12756. Two conflicts, both from my "Don't look up custom element definitions for sanitized-away elements" commit (fab5f2b) and this PR independently factoring sanitize into an element-name algorithm. Resolved as follows:

• Kept this PR's "sanitizer action for element name" and dropped my "get the sanitizer action for an element name" and the sanitize wrapper, since this PR removes that algorithm and moves the sanitize dfn to the intro prose.
• In "create an element for a token", merged this PR's steps (switch document to the template contents owner for dropped elements, sanitize the is attribute) with my step that skips the custom element definition lookup for dropped elements. Both needed the action, so I compute it once into sanitizerAction and use it in both places.
• My Noah's Ark note linked to "sanitize attributes", which this PR removes. I repointed it to "create an element for the token" and reworded the rationale, since removed attributes are now never appended to the element rather than removed afterwards.

Otherwise the diff is unchanged from before the rebase.