Question

What codecs will be supported with the HTML5 video tag?

Will it vary by browser, or is there a spec of specific codec that will be supported?

Answer 1

According to wikipedia:

The current HTML5 draft specification
does not specify which video formats
browsers should support in the video
tag. User agents are free to support
any video formats they feel are
appropriate.

HTML5 is in draft format though and this may change.

There is an ongoing debate about the suitability of various algorithms and it seems that currently not everyone involved is able to agree on an answer. The three major contenders are Ogg/Theora, H.264 and VP8/WebM. The present state of the various implementations varies between the browsers.

The current HTML5 draft spec states:

It would be helpful for
interoperability if all browsers could
support the same codecs. However,
there are no known codecs that satisfy
all the current players: we need a
codec that is known to not require
per-unit or per-distributor licensing,
that is compatible with the open
source development model, that is of
sufficient quality as to be usable,
and that is not an additional
submarine patent risk for large
companies. This is an ongoing issue
and this section will be updated once
more information is available.