You can try it on this test page:
http://visitmix.com/LabNotes/HTML5-video-tag-with-H264-codec

This script serves to show how HTML5 needs to support open codecs not H264, so that we don’t continue to be locked into Flash. Hopefully, Google will open source the recently purchased On2 codecs.

Also any expert in video codecs will tell that theora simply cannot match h.264 in quality, or even probably vc1. This based on the simple fact of what avoiding patents allows. It’s not what it seems like at the current state but what is technically possible with the technologies implemented in theora. It simply cannot match h.264 in any objective sense. What I’ve heard is that maybe it can get up to the quality of a ASP encoded videos.

As for CPU decode time, that is a major issue that the Mozilla team and developers of other browsers are going to have to work on. As is, it’s more efficient to decode h.264 through flash than theora through the video tag. These developers are leaping into the world of video playback and display – something they probably don’t know much about. It’s hard enough to get video to display properly in a dedicated player. It’s almost akin to how the ogg developers are coming from the world of audio encoding and are now dealing with video encoding.

The web has been this successful because it started free and open. I firmly believe that Theora would be the only sensible choice for the web, even if it were only comparable to obsolete patented codecs (which it isn’t). The reason I bother to post, however, is because of the glaring inaccuracies in the comments so far. It is also worth remembering that there is a lot to be gained financially by those supporting H.264, if it is accepted as a standard. Also, some companies involved in the debate will already purchase so many licences for H.264 that additional per-unit costs for licences are not as high. See:
http://www.mpegla.com/avc/AVC_TermsSummary.pdf
As pointed out, these terms could completely change in the future, especially if the web comes to rely on H.264.

The fairest comparison page that I have found is this:
http://people.xiph.org/~greg/video/ytcompare/comparison.html
(also discussed here: http://people.xiph.org/~maikmerten/youtube/ ).

As somebody has already pointed out, a lot of recent work (for example Thusnelda) has gone into improving Theora quality, so older encoders are no longer representative. There is also huge room for improvement, as I understand that a key design point of the codec was that huge advances could be made in the encoding without changing the decoder (ideal for embedded devices). A very honest (and now somewhat out of date, with the recent improvements to Theora) critique of Theora was provided here:
http://web.mit.edu/xiphmont/Public/theora/demo.html
which concluded:
“This document […] says the current encoder is lacking compared to the very very best. It certainly is. Yet, the Theora format is entirely capable of closing with H.264 and MPEG-4 in terms of R-D while still requiring a fraction of the CPU time.”

Reduced playback demands seem important to me, given the increasing trend to browse the web and explore rich content on small devices.

Please stay strong on this, Mozilla!

Basically they don’t trust external codecs, and with good reason.

I don’t think anyone is happy with the way things are now, but the reasons the Firefox doesn’t support H.264 seem to me to be better than the reasons Safari doesn’t support Theora.

Can’t be done. That’s a license violation, period.

BTW, wasn’t the supposed bad quality of Theora the whole reason work started on Dirac? It seems to me that the Ogg/Dirac combination could compete with MPEG-4.

BTW, considering Microsoft isn’t coming to this party, cutting Mozilla out is a VERY BAD IDEA, considering they represent the largest group of Internet users after IE users. If neither IE nor Firefox supports MPEG-4 video in the video tag, no one will use it, Safari support or no.

It’s time to bit the bullet, Firefox. Build in h.264 support and offer the download from servers in a country without software patents — much like what was done back when browsers with 128-bit encryption were considered illegal in the US.

Patents are BS to begin with, and it’s time to route around the damage.