Cascading Style Sheets control the user interface of the site [to a degree] and we believe that to keep life simple users should have a consistent interface. You don't want visitors to `learn` how to use your site.
We use two: Materialize by Google, which is being used on this site and Bootstrap by Twitter. Both of these frameworks are very popular and well supported. At the time of writing, we prefer Materialize, but that's just because it's a little bit more recent and the developers have quite clearly addressed a lot of criticisms with its big sister BootStrap [V4 is a good choice too].
Note | Google's Materialize is by default jQuery based and A2 obviously doesn't play nice with jQuery, but there is a full npm for Materialize so this is not a barrier to its use.