Your EPUB Is Fine. Kobo Disagrees. Blame Adobe

Ew. I didn't think they pulled in javascript in practice.

It's optional to support but was promoted to a "SHOULD" in 3.3 and is way more restrictive, you may not even be able to modify the DOM, or cause a content reflow. The ones I've seen that has it don't have a ton of performance, combined with the limitations it's not that easy to actually use in practice unless you're willing to limit which renderers you support. It's mostly useful for more experimental zines.

That's a good point. I was thinking they were frozen at XHTML and CSS 3, which is much easier.

It does mandate the XHTML flavor of HTML so they don't have to deal with the parsing bullshit, and warns that not everything is supported in CSS. You barely have to implement anything CSS-wise to be spec compliant.