[DiapDealer]

I'm not following very well. The IDPF's Epub Checker cannot be influenced to treat an EPUB2 like an EPUB3 or vice versa. EPUB3s are validated against EPUB3 spec, and EPUB2s are validated against EPUB2 spec. It determines which spec to use by checking the version attribute of the OPF's package tag (which determines whether an EPUB is an EPUB2 or an EPUB3).

Also ... HTML5 is not supported in EPUB2. Not sure why you're referencing it with regard to an EPUB2.

I'm also not certain what any of this has to do with Sigil (except for the fact that you created your test EPUB with Sigil). Why do you believe EpubCheck is validating your EPUB2 using the EPUB3 spec in the first place? I've never known it to get that wrong.

The fact that Kindlegen/Preview is OK with an EPUB, or that it "works" in ADE has absolutely nothing to do with whether or not it meets EPUB specifications.

I can tell you that advanced list manipulation in an epub isn't worth the trouble. Trying to alter the starting numbers or the bullet type will work on one device/app but not another. There is no consistent support for lists other than ones that start with the number 1, or that use a standard bullet. Fighting to get anything else to consistently work across a wide range of devices will only drive you nuts.

Regardless of where it "works", the strike tag is obsolete in XHTML (which is what Sigil and EPUB2 uses).