How to prevent negative SEO impacts caused by poor HTML quality

The question rises over and over again: whether and how could HTML markup negatively impact SEO. Googlebot is indeed a smart HTML interpreter:

• it has a high tolerance to HTML syntax errors,
• it doesn't force websites to comply with W3C validation rules.

But nevertheless there are some HTML misuses, which could painfully hurt SEO. To begin with this topic i relate to a pair of posts by two respectable Googlers and by commenting the posts i list HTML issues causing negative SEO effects:
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