Do they? Maybe they don't? Maybe we're at a point where people can author html/css, and if there are syntax issues, they just 'break' until someone fixes it. 25 years ago, maybe we needed some laxness, and perhaps today, we don't?
If your browser handles all of the popular sites, people will use it. Nobody cares about whether Flash loads anymore, do they? Yet there's millions of Flash animations and games never ported to another platform
In the worst case, the browser would then have to download some additional code that contained all the support for invalid HTML/CSS code. In the best case, the 25 year old page would be served with a header containing a cryptographic proof that the site really had been around for 25 years, and it wasn't just some newly created attack site that was exploiting some weird behaviour in old renderers.