First, if you just want to embed videos using the html5 `<video>` tag,
you can do that without switching anything else to html5.
However, if you want to fully enter the brave new world of html5, read on..

Currently, ikiwiki does not use html5 by default. There is a `html5`
setting that can be turned on, in your setup file. Rebuild with it set, and
lots of fancy new semantic tags will be used all over the place.

You may need to adapt your CSS for html5. While all the class and id names
are the same, some of the `div` elements are changed to other things.
Ikiwiki's default CSS will work in both modes.

The html5 support is still experimental, and may break in some browsers.
No care is taken to add backwards compatibility hacks for browsers that
are not html5 aware (like MSIE). If you want to include the javascript with
those hacks, you can edit `page.tmpl` to do so. 
[Dive Into HTML5](http://diveintohtml5.info/) is a good reference for
current compatability issues and workarounds with html5.  And a remotely-loadable
JS shiv for enabling HTML5 elements in IE is available through [html5shiv at Google Code](http://code.google.com/p/html5shiv/).

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Known ikiwiki-specific issues:

* [[plugins/htmltidy]] uses `tidy`, which is not html5 aware, so if you
  have that enabled, it will mangle it back to html4.
* [[plugins/toc]] does not understand the html5 outline algorithm.