Thanks for drawing attention, but we already have a lot in motion for v11, and anyone may use any CSS frameworks they like as things currently are within their own themes.

There's a lot more to this than just adopting a particularly library that people like. These libraries tend to go out of fashion very quickly which would have a severe impact on us if we went too deep into a rabbit hole coding to the oddities of a specific library across a thousand templates, and we've learnt the hard way with a project that they can make the CSS so complex that nobody but a professional web designer can work with it. And even just upgrading to a new version of a library can be a massive overhaul, e.g. Bootstrap switched from LESS to SASS. We might do some kind of dual approach where we make some further improvements to the default theme (which will also have responsive design) and stay relatively simple, and then some very complicated official theme that has an advanced framework but is too complicated for regular users to be expected to change.