Google doesn't use WC3 compliance as a ranking factor.
That said, there are cases where badly botched HTML will cause issues: e.g. META description / TITLE not properly picked up by Googlebot due to malformed code.
Best practice is to run a site through a validator at least once. No need to fix stuff like & in URLs just make sure you got no major issues (e.g. screwy HTML nesting, unclosed </a>).
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