Add display:contents support: elements with this display value generate no
box but their children participate in the parent's layout. Fix the HTML
parser to create an implicit <head> for head-only elements (title, style,
link, meta, script), preventing <title> text from rendering visibly in
the body. This was the root cause of most WPT reftest mismatches.
- Add Display::Contents variant and CSS keyword parsing
- Add build_children_into() to flatten display:contents children
- Fix implicit <head>/<body> insertion in HTML tree builder
- Promote 53 tests known_fail→pass, demote 5 false-pass tests
- Regenerate 4 golden expected outputs for shifted node IDs
Net result: 156 → 204 passing WPT tests (+48).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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