Because HN's development prioritized "good for the user" over "good for browser developers".
https://www.w3.org/TR/design-principles/#priority-of-constit... https://datatracker.ietf.org/doc/html/rfc8890Anyway, the "errors" that OP mentions are simply just deprecated attribute styling and like, one or two instances of center tags. Hardly breaking anybody's back to continue supporting those.