I was disappointed to find the Scrivener doesn’t yet support HTML5 as an export option. I haven’t used HTML 4.x for quite some time.
It does support XHTML/Transitional as well, but I think it might be set to HTML 4 by default (not sure why that is come to think of it). The setting is in the “Export Tab” of the Import/Export preference pane, Document type.
As for why we don’t support HTML5 overall, ask Apple, they provide the import/export features of the text engine on the Mac. I mean that rhetorically though, actually sending them feedback is probably a waste of time—they haven’t made significant updates to the core text engine in nearly a decade—but try to find a replacement text engine with better features and typesetting.
Well, if you export via Multimarkdown which Scrivener supports well, then you can get excellent HTML5 output, and can switch to Pandoc or other text engines for very flexible outputs.