> no cookie popups, no "subscribe to our newsletter" slide-ins, no phenomenon where the content loads but then a second later the page resets and re-loads the content again (probably with more ads or something)
to fix rendering layout using tables and cell width to render comment levels.
Just a few lines of CSS and Arc really. That would be a big overhaul only because HN is very small, but not that big in absolute.
Anyway, 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.