>The image tag has worked fine without a "baseline" image format since the dawn of the web.
IE's historically poor support for PNG is probably the main reason so many sites still use GIFs where PNGs would be better. (Edit: And for that matter, as others have pointed out here, PNG was invented because the owners of the GIF patents threatened to sue the entire Internet.) Browser support for TIFF is an inconsistent mess.
In the case of the img tag, the most common browsers all supported two common formats pretty well (JPEG and GIF), so those two formats became the de-facto baseline image formats organically.
I think the situation with video is similar. If Firefox, Chrome, and Opera (the influential browsers used mainly by the early-adopter, techie crowd) all support one specific codec for HTML5 video, that will become the de-facto baseline video format organically.
IE's historically poor support for PNG is probably the main reason so many sites still use GIFs where PNGs would be better. (Edit: And for that matter, as others have pointed out here, PNG was invented because the owners of the GIF patents threatened to sue the entire Internet.) Browser support for TIFF is an inconsistent mess.
In the case of the img tag, the most common browsers all supported two common formats pretty well (JPEG and GIF), so those two formats became the de-facto baseline image formats organically.
I think the situation with video is similar. If Firefox, Chrome, and Opera (the influential browsers used mainly by the early-adopter, techie crowd) all support one specific codec for HTML5 video, that will become the de-facto baseline video format organically.