HTML / XML / JS, low-level validity (cross-cutting)MediumInferred
Invalid/unclosed HTML breaking parse
Badly broken HTML, unclosed tags, invalid nesting, can make the browser and crawler build a different DOM than I intended, dropping or misplacing content. Valid HTML keeps the parse predictable.
What it is
Malformed markup.
Why it matters
Parsing errors can hide content or directives.
How to fix it
Validate and fix HTML.
How to find it on your site
- Run the page through the W3C validator.
- Fix unclosed tags and invalid nesting.
- Re-check the rendered DOM matches the intent.
- Add validation to your workflow.
Cross-reference to ranking and citation factors
Severe HTML errors can corrupt the parsed DOM and hide content. Valid markup keeps what you wrote intact.
Impact
Low-medium. Inferred.
Evidence
Google tolerates imperfect HTML but parse-breaking errors can hide content. Google Search Central, Understand JavaScript SEO Basics; Google Search Central, In-Depth Guide to How Google Search Works
Sources