However, Bruce Lawson, Web evangelist at browser maker Opera, says it's not clear that such a takeover is wanted or needed. Flash works across all Web browsers, and programmers only need to write the code for the video once, instead of tailoring it to the capabilities of different browsers, he notes.

In addition, says Lawson, Flash is much better at letting developers use digital rights management software to keep the videos from being downloaded and distributed without permission. "If you're a company showing video on the Web, and it's important to you that that video can't be downloaded, captured, or distributed, then Flash is the tool to use," he says. So far, Lawson adds, he knows of no plans to extend similar digital protection capabilities to HTML5 video.

One feature that Sublime Video is trying to give HTML5 video is a full-screen mode--the absence of which is a glaring difference between HTML5 and Flash. But Michael Smith, who helps develop the standards for W3C, says there are serious security issues involved in allowing video within a browser to be full screen. For instance, a malicious hacker could provide a link that seems familiar and safe, but that takes a user to a full-screen video that mimics her desktop and a fake bank log-in designed to capture credentials. "For security reasons, you need to keep users aware of the fact that they're watching a video," Smith says. The group is working to develop a design that enables a full-screen mode while still being secure, he says.

Another feature that's not yet fully fleshed out in HTML5 is closed captioning and subtitles via synchronized time pegs. This is available with Flash, although Opera's Lawson notes that it's still difficult to get text data out of a Flash player and make it useful for something like search.

Smith stresses that HTML5 is not being developed to replace Flash, and it's not a matter of matching the technology feature for feature. "It's not about replacing perfectly good, working technologies," Smith says. "What we're trying to do is provide choices."