Playground HTML fixes

[Potherca]

I just discovered H²Space and I think its awesome!

Obviously I headed straight over to the playground to mess around with things. I noticed some of the HTML was a bit wonky, this MR tries to correct some things.

Specifically:

• An HTML document really should have a lang attribute.
• A character-set should be declared to help browsers display text correctly
• A title is also nice
• <pre> tags should not be wrapped in a <p>. As they are both block-level elements, and <p> is self-closing, opening a <pre> effectively closes <p>, meaning that browsers think this page has a lot of random </p>'s (and is missing some <p>s as well).

Hope this helps, keep up the good stuff!

[sbazerque]

Guilty as charged on the wonky HTML, thanks for the fixes @Potherca !!!