Every ad requires a JS click tag, which is pretty standard code 95% of the time. The other 5% of the time an ad platform will have special click tag requirements with only vague instructions, so some JS knowledge will be helpful.
Sometimes you'll want to use a Hype extension in order to pull off something that Hype can't do natively. This isn't at all common (for me), but a recent example is when I needed to include color shapes that used a multiply blend-mode. There's a JS extension for that purpose, available within the forum.
Even though you mostly won't be CSS hand-coding, it's valuable to know it. It's not uncommon for mobile banners in the 320×50 size to be limited to 50KB. If you want to pack enough retina images in there, animating with CSS instead of Hype will save you from the file weight of Hype's JS file.
The most common file size limit for most digital ads is 200KB. Unless you resort to visual trickery, you can assume 3D is off the table for any digital ads. As to images and file size, the general rule is that JPGs are best for photographic images, PNGs are best for vector graphics. It's not uncommon to be forced to use PNG for silhouetted photographic images. Larger image size, but unavoidable since it needs to be silo-ed.
You'll also become very familiar with overcoming browser quirks. The biggest one (to me) is that Firefox renders subtle movement in a choppy manner. Lucky for all of us now, Internet Explorer is a thing of the past. For the most part.