Dig up any HTML validation errors in generated HTML
This was suggested by karsten in #21166 (moved). An answer on StackOverflow details a bookmarklet for validating the generated HTML as is currently loaded on the page. I think it's worth digging into this as we've seen in #21166 (moved) and #21234 (moved) that mistakes can slip through.
The bookmarklet is here:
Once digging is completed, any issues found should be filed as tickets (preferably with patches for review).
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