Positional HTML methods

[noamr]

Add appendHTML, prependHTML, beforeHTML, afterHTML, and replaceWithHTML, alongside their *Unsafe variants.

Use 3 helper algorithms for HTML insertion (pre-insert/replace/set), and call those from the appropriate web-exposed method.

ChildNode methods fail silently when the child has no parent, like their equivalent DOM methods,
and fails with a HierarchyRequestError for an invalid parent. Note that a direct child of a template element is considered an invalid parent, as setting HTML to template elements would affect its content rather than its children.

Renamed "set and filter HTML" to "filter and set HTML" to be more consistent with the order of operations and with the newly added helper methods.

Closes #10122

• 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 positional HTML setting methods web-platform-tests/wpt#60555
• 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]

---

/dynamic-markup-insertion.html ( diff )
/infrastructure.html ( diff )
/system-state.html ( diff )

[zcorpan]

LGTM % typo

[noamr]

Fixes from Opus review:

Blockers

• Trusted Types sink names: Updated sink names to use the declaring interface ("ChildNode beforeHTMLUnsafe", "ChildNode afterHTMLUnsafe", and "ChildNode replaceWithHTMLUnsafe" instead of "Node ...").

Suggestions

• DOM divergence note: Added an explicit non-normative <p class="note"> under parent for HTML insertion explaining the intentional divergence from DOM before()/after()/replaceWith() regarding template and Document parents.
• domintro error descriptions: Updated the domintro entries for all six positional methods to explicitly mention throwing HierarchyRequestError if the parent is a Document or template element.
• Helper invocation phrasing: Standardized algorithm step invocations across all call sites to use "the parent for HTML insertion given this" and "the HTML insertion target given target".
• Method alignment: Aligned the step structure of replaceWithHTML and replaceWithHTMLUnsafe (binds parent, returns early if null, then invokes filter and replace with HTML).
• [Unscopable]: Omission of [Unscopable] for the new ChildNode methods follows standard Web IDL practice (retained only for legacy identifiers for compat).

Nits

• Helper parameter signatures: Standardized to a string <var>html</var> and data-x="tt-trustedparseroptions" across filter and set HTML, filter and pre-insert HTML, and filter and replace with HTML.
• Formatting & Whitespace: Fixed indentation (3 spaces) and line wrapping in parent for HTML insertion, closed missing </p> tags on method steps are:</p>, removed double spacing in algorithm signatures, and removed the stray extra blank line before the Document.parseHTML domintro.