Javascript or CSS

As a general rule, always use CCS first. The animations within CSS are much more efficient on power and much faster than those of JavaScript. It also takes less resources to load and perform CSS than it does JS. CSS goes hand-in-hand with HTML and styles the page; JavaScript is how the page behaves.

If you can't achieve what you need in CSS, then and ONLY then use JS. It's simpler to read and maintain when things like <a> hover states, colors etc are coded in CSS.

I pretty much use JS for backwards compatibility between browsers, images sliders, galleries etc. I never use it for structure and basic animations. CSS3 adopts new features pretty frequently.

One last thing, you don't have to worry too much about this now but it's still a good to be aware of this. Users have the option to disable any JavaScript from loading from any page. If all the functionally and aesthetics is coded in JS, you just lost all of that hard work. It was a big deal in the 90's and early 2000's but not so much now.