HTML5 Video doesn't play

You need to have GStreamer plugins installed which implement the codecs.

1. You need gstreamer0.10-pulse if you're using PulseAudio.
2. You may need gstreamer0.10-alsa for ALSA, depending on your distribution.
3. With Arch Linux, you may need to install liboil explicitly if it is not installed already (see this bug report: https://bugs.archlinux.org/task/20945).
4. You need plugins for Theora, gstreamer0.10-base and MPEG-4 incluing aac (e.g. gst-plugins-faad), gstreamer0.10-bad. For WebM, you'll need plugins for vorbis (-base), matroska (-good), and vp8 (-bad). Have a look at http://www.gstreamer.net/documentation/plugins.html for details.
5. For Youtube or Vimeo, you need WebKitGTK+ 1.1.20 or newer.
6. You can test your installed codecs here: http://ie.microsoft.com/testdrive/Graphics/VideoFormatSupport/Default.html
7. Since Midori 0.3.5 you can look at “about:version” to see which video codecs you have installed.