So far, one of the main arguments against widespread implementation of HTML5 video has been the uncertain licensing future of the H.264 standard. Proponents of Flash video and organizations committed to license-free software, like the Mozilla foundation, said that while H.264 was currently royalty-free (and would remain so until 2015), there was no guarantee that MPEG LA wouldn't start charging licensing fees later on.
In that event, if HTML5 had supplanted Flash as the de facto standard for video on the web, it would have meant that organizations and possibly even end users would have found themselves saddled with onerous fees after 2015.
That theoretical stumbling block has disappeared. MPEG LA has announced that H.264 will be royalty-free forever so long as video encoded with the standard is free to end users. This means sites like YouTube and vimeo will never be charged licensing fees to serve video on the web; presumably, it also means that Apple will continue to pay licensing fees to sell videos in the iTunes Store.
Mozilla's Firefox browser doesn't currently support HTML5 video (via H.264, that is -Ed); the uncertainty of H.264's licensing future meant Mozilla wanted to stick with Ogg Theora, a video codec Mozilla believed would be unencumbered by patenting issues. With MPEG LA's announcement that H.264 will be royalty-free in perpetuity, it's likely only a matter of time before Firefox joins browsers like Safari, Chrome, and Internet Explorer 9 in fully supporting HTML5.
This is good news for almost everyone except Adobe. Adobe's main argument against moving away from the current Flash-dominated web video landscape to one with a truly open standard like HTML5 is now invalid. While Flash may continue to hold onto its grip on interactive web content, MPEG LA's announcement likely points to an end to Flash's dominance in video. This is also the last nail in the coffin for any possibility of Flash running in iOS -- with possibly the biggest obstacle to widespread implementation of HTML5 video now gone, there's zero incentive for Apple to hitch its wagon to Flash.