Epub 3.x does support a substantial subset of HTML5. However, there are only a small number of core media types that can come with an expectation of working.

Atop of that, most styling can be expected to be ignored.

The CSS-section of the spec [0] lays out most CSS fits the spec, with a few exceptions, and a few additions, but that most readers won't make use of them. That is absolutely the case.

Most of the speed from an ereader over a browser comes from these parts:

+ You can safely ignore most CSS, or all CSS and just use Author-Defined CSS profiles.

+ You can safely ignore any of the script tags you encounter, though they are part of the spec.

+ You don't need to support every media-type out there.

+ Most documents won't be HTML - they'll be the well-defined XHTML, meaning you need less cascading rules to transform the document into something well-formed.

[0] http://www.idpf.org/epub/31/spec/epub-contentdocs.html#sec-c...