Stream HTML into element or ShadowRoot

[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.

The HTML fragment parser is divided into two:

• Preparing the fragment parser (create an inert document with a proper element stack)
• Using that parser to parse into a DocumentFragment

The preparation algorithm is used when streaming, by creating a new fragment parser, and then the stream is routed to feed that parser.

When parsing, the "insertion target redirection map" is configured to reroute insertions intended for the root to go directly into the stream's target element, instead of going into the DocumentFragment.

A special sanitizer mitigation is in place to avoid these removed elements from incurring side effects, by creating them in an inert document if they are about to be removed.

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 )
/infrastructure.html ( diff )
/interactive-elements.html ( diff )
/parsing.html ( diff )
/references.html ( diff )
/system-state.html ( diff )