Are there any cases of browsers breaking on older web standards? The only things I can think of that ever broke are plugins (Java, Flash) and vendor-specific extensions (e.g. lots of IE quirks).
<blink> element that was deprecated by browsers maybe. But that was non standard to begin with.
Probably only things experimental, non spec are in danger, but it's safe to think css is very future proof compared other languages designed for layouts.
I think chrome and safari made a bit of a mess with the rounded corners before it was standardized. One of them dropped the browser prefix and ended changing their implementation I think. My memory is very hazy though.
