Sudoku
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Modernized HTML
Got rid of those self closing tags from the XHTML days.
Removing the closing slashes in empty elements isn't modernizing; that's actually an original HTML feature for back when every bit counted, and the world's fastest modem ran at 14.4Kbps; this saved a little bit of download time at the sacrifice of a more ambiguous syntax. If you want to conform to the XHTML form of HTML5, the closing slashes should be kept.
The rest looks nice, though