Clean up the markup

I’ve mostly applied the things described here: http://mathiasbynens.be/notes/html5-levels

* Removed useless XML prologs from HTML files
* Switched to the `<!DOCTYPE html>` doctype
* Switched to `<meta charset="utf-8">`
* Removed `type="text/javascript"` from `<script>` elements
* Removed `type="text/css"` from `<style>` and `<link rel=stylesheet>` elements
* Removed unneeded explicit references to `/favicon.ico` (http://mathiasbynens.be/notes/rel-shortcut-icon)