For companies that compete with Microsoft, HTML5 is almost the Holy Grail, offering the ability to run applications regardless of the underlying operating system. While the browser isn't more important than operating system today, Google this week firmly suggested it is only a matter of time.

We've heard that story before. Java was supposed to raise apps above the level of the operating system, offering cross-platform "write once, run everywhere" applications that would break the coupling between an application and a specific operating system. Proponents predicted Windows would become less important with the rise of Java apps.

While Java has accomplished a great deal, it's potential as an OS-killer has not been realized. HTML5 has a better shot.

At its developer conference this week, Google demonstrated HTML5 applications support inside future versions of its Chrome browser and the future Android 2.0 operating system. Mozilla executives also promised HTML5 support inside the forthcoming Firefox 3.5 browser.

Google demonstrated how HTML5 allows tighter integration of browsers and applications, such as its Google Web Elements. Developers will be able to add applications to web sites by adding only a few lines of HTML5 code, much as they already do with Google Web Elements.

Microsoft, meanwhile, has announced plans to support HTML5, but appears to be keeping it as arm's length, at least for now.

What does this mean for users?

HTML5 is a standard that is still being developed and is likely to remain so for several years. Its focus on running applications within the browser is an important driver of interest in cloud computing, where applications live somewhere off on the Internet and are delivered by the browser.

The focus of future browsers will shift from "going places" to "doing things." This will be a boon to free operating systems, which will increasingly be able to hide themselves under the browser user interface. While Windows and Mac OSX won't go away overnight, the pressure on them will be to innovate beyond the browser, perhaps through a common set of extensions for HTML5 applications to use.

It is too early to start betting against desktop operating systems from the major vendors. However, it is clear their role and importance is likely to change--and probably diminish--as browsers become dominant in users' lives.