This is based on https://github.com/kangax/html-minifier/blob/6b2d4536d82819143b468b41a89c700b6c61631f/src/htmlminifier.js#L197 and
spec from https://www.w3.org/TR/html51/single-page.html.

Couple of other changes to tests due to support update:
- autobuffer has been dropped in favour of preload attribute, ref: https://msdn.microsoft.com/en-us/library/ff974743(v=vs.85).aspx
- pubdate attribute has been dropped from spec, ref:  https://www.w3.org/html/wg/tracker/issues/185