A patent on web browser technology held by Eolas Technologies has been invalidated by the U.S Patent and Trademark Office (USPTO), which recognized arguments from the World Wide Web Consortium and others that existing “prior art” should nullify the patent.

If the USPTO decision survives an expected appeal, it will likely void a $521 million jury award against Microsoft for infringing on the Eolas patent with features of its Internet Explorer browser. It also spares Microsoft the need to make modifications to its Windows operating systems and IE browser to allow them to continue to use popular multimedia plugins from Apple, Macromedia, Real Networks and Adobe. Microsoft outlined the planned changes last year but put them on hold in late January as it awaited a ruling from the patent office. The ruling would also avert the need for developers to modify millions of web pages using the HTML tags APPLET, OBJECT and EMBED, which would have been affected by the patent ruling.

The patent in question is held by the University of California and licensed to Eolas Technologies. It covers systems allowing browsers to “access and execute an embedded program object,” and is based on work by a Cal team led by Michael Doyle. This “plugin” concept is now widely used to display multimedia within a browser window.

The concept was widely discussed at the time on the www-talk mailing list hosted by Web creator Tim Berners-Lee, as well as by Dave Raggett in the HTML+ specs he authored in 1993-94 for the Internet Engineering Task Force (IETF).

Nonetheless, the University of California’s 1994 patent application for the technology was approved by the USPTO in 1998. Microsoft noted that the U.S. Patent and Trademark Office has only invalidated 151 patents out of nearly 4 million awarded since 1988. That doesn’t reflect patents that have been invalidated by outside court proceedings.