HTML / XML / JS, low-level validity (cross-cutting)MediumInferred
Invalid/unclosed HTML breaking parse
What it is
Malformed markup.
Why it matters
Parsing errors can hide content or directives.
How to fix it
Validate and fix HTML.
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