HTML5 got one thing right though: standardization of the DOM failure behavior. As an implementation detail of their design, they went with "sensible recovery" for failures over stricter failure modes.
It was browsers that did that in the first place. HTML5 just standardized the exact behavior on failures.
Incorrect. HTML5 synthesized the exact behavior that was closest to the majority of browsers. But not all browsers agreed (e.g. Mozilla would change its HTML parsing behavior depending on network packet boundaries), so there was still effort aligning with the newly-specced common parsing algorithm. At the time there was much skepticism that such alignment was even possible.
HTML 4 - vendors implemented the spec incongruently and failed in their own special ways. XHTML strict - standard parsing rules with strict failure mode. HTML 5 - standard parsing rules, suggested (but not required) rendering behavior for browser uniformity, and well-defined failure behavior.
It was browsers that did that in the first place. HTML5 just standardized the exact behavior on failures.