How much of a browser's time is spent in JS vs. rendering? How much overhead is spent of parsing/rendering? JS isn't even 25% of overhead in most sites, or even dynamic applications. Rendering of reflows, and other UI elements is. Understanding this in the scope of JS is important, as this is where it gets triggered. Hell, having something like AngularJS out of the box in the browser earlier on would have helped a lot.

Just the same, it started with well-formedness being loose, and continued from there.

Sure, slow rendering/layout/performance would be a perfectly reasonable complaint about the web platform.

The well-formedness thing is, sorry to be blunt, a totally crazy thing to complain about.

Every single platform that has any popularity introduces rough edges like this over time. It's impossible not to because every single bug that introduces relaxations gets baked in as content comes to rely on it. It is impossible to ever remove those relaxations, and really, it's totally fine.

There is a cost to lack of well-formedness, but on the list of problems with the web it is waaaaay waaaay down there.