Always produce HTML5 doctype and new attributes, but not new elements
According to caniuse.com, a significant fraction of Web users are
still using Internet Explorer versions that do not support HTML5
sectioning elements. However, claiming we're XHTML 1.0 Strict
means we can't use features invented in the last 12 years, even if
they degrade gracefully in older browsers (like the role and placeholder
attributes).
This means our output is no longer valid according to any particular
DTD. Real browsers and other non-validator user-agents have never
cared about DTD compliance anyway, so I don't think this is a real loss.