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CSS is actually for specifying formatting without cluttering up the source HTML with extraneous markup. You can autogenerate some extra text with it, but it's poorly supported even in web browsers for that and not recommended. Consider doing a navigable table of contents and NCX chapter marks instead.
There is currently no container version 2.0, but the spec is built for forward-compatibility and backward readability thus assumes there someday might be. You may be able to get away with omitting for some software but there will probably be error messages or outright refusals to open in others, such as KindleGen when converting to mobi.
ePub potentially accepts a very wide range of audio/video/image formats just the same as HTML. Again, the limitation is the software and modern readers support jpg and gif. You can actually get better compression/smaller filesizes with png than gif if you use palettes and get the settings right in many cases. SVG is just glorified text and compresses very well; the problem is support.
Hope this helps.
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