Use defensive CSS to renforce accessibility

[ffoodd]

By using stronger selectors like html[lang] or img[alt], we could improve developers' accessibility knowledge:

• by making styles dependent of an accessible markup,
• thus requiring developers to use our HTML templates.

This has many downsides though:

• everything should be explained, at least in the documentation;
• those selectors might only be compiled for non-minified versions (should be equal to dev mode, shouldn't it?)

I guess the best ressource for now is a11y.css and its wiki.

FWIW, Dan Eden implemented an assert-selector mixin in Scooter to do just that; we may try to include this in non-minified styles only.

Since Boosted assumes its accessibility, I guess it could be done :)