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 Foundation by Zurb. Both of these frameworks [libraries] 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 mainstream and the developers have quite clearly addressed a lot of criticisms with its rival sister BootStrap [we can use Twitter's Bootstrap if you insist].
If you like your web-sites to be `bouncy` or some might say `animated` then you can go as far as reasonable taste allows with animate.css and something called Tweening; this is very much a personal choice and can work well, but these things do come and go in fashion/design terms.
Here is some example CSS - just mouse-over the below: