Video / Audio presentations for web: formats , accesibility concerns, file sizes

Hello,

I understand that its always better to refrain from multimedia presentaions
in web such as video and audios.

But, there is a requirement in our project to present both video and audio.
(Example: www.espn.com - Loads the video)

One more requirement is that the user can also download video and audio
files to use it as podcasts.

The main objective is to present video and audio in a universal format
(PC/Mac/Podcasts), small size (easily playable and downloadable) and also
taking accesibility standards in web (If the users would like to view it or
hear it on the web)

Now, I am trying to find  research / information on
-  Video/Audio formats, recommended formats compatible for both PC / Mac
without any 3rd party player downloads
-   maximum allowable size for low-high bandwith users
-  Accessabilty standards (like captioning/subtitles and other alternative
text supplements)
Do you know where I could get the stats / research / information / samples
on this?

Also if you have any other suggestions for presenting video/audio on the
web, please email directly to the group.

Thanks, Ravi

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