Link targets need to be focusable to work with VoiceOver.
Making <main> focusable causes some side-effects, like making the TAB
key go to the beginning of <main> instead of the element after the
currently-clicked region.
Also removes the annoying outline around "main" in some non-mainstream
browsers, without having to add extra CSS.
The site now has polygot markup and can handle both XHTML5 and HTML5
parsing rules. My staging site will be XHTML but my main site will be
HTML5, just in case of parse errors.
If other tools (e.g. LightHouse) end up supporting XHTML5, I'll consider
switching the content-type to XHTML.
