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.
So same question then -- previously compatible javascript breaking in newer browsers is a thing? Are there some common examples of this (e.g. not just edge cases).
This is not relevant. We are considering the interaction between our code and the library. If the library has breaking changes to its api our code will no long work with it when the library needs to be updated to the new version and our site will break until our code is changed...
Fair enough. Your question makes sense then. The trouble is without security updates there is bound to be some exploit that will go un-patched so when support for bootstrap 3 finishes we will have to upgrade to 4 whether we like it or not.