index.html: switch to HTML5.
README.md: use the new GitHub pages io URL.
Update gh-fork-ribbon.css.

* add missing fallback border-color
* remove unit from 0 and change `0.00` to `0`
* add missing vendor prefixed properties
* use the shorthand font property
* fix typo in comment
* use the hex colors consistently
index.html: minor cleanup in the inline CSS.