Detailed answer
Reduce the problem to a small example and run it through an HTML checker.
Fix structural errors from the top, because one malformed tag can cause several later messages. Compare source with the parsed DOM and retest after changes. Validation checks syntax; it does not prove accessibility or complete application correctness.
Example
<!doctype html><html lang="en"><head><title>Validation example</title></head><body><p>A complete paragraph.</p></body></html>
Understanding the behavior
Start with the smallest failing document and fix structural errors before chasing visual symptoms. Browser recovery can make invalid source appear partly functional, so compare source, parsed DOM and validator messages. The first structural error can cause several later warnings.
Practical implementation notes
Reproduce the issue after each meaningful fix. A validator checks markup conformance, but it does not establish correct business logic, accessible interaction or a useful visual layout by itself.
How to check it
Fix the first structural problem and rerun the checker before editing every later warning.
Expected result
Cascading messages can disappear when the underlying markup is corrected.
Watch out: Several validator messages can have one root cause.
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