A federal appeals court on Wednesday agreed to give Microsoft Corp. a second chance to prove that it did not infringe on a browser-technology patent, and that it shouldn't have to pay more than $520 million in damages.

Under the ruling by the federal appeals court in Washington, D.C., the case was sent back to the U.S. District Court in Chicago, where Microsoft would be given the opportunity to prove that the technology owned by the University of California had not been invented by the patent holder, Microsoft and the plaintiff's attorney said Wednesday.

Microsoft in a statement said the decision was a "clear affirmation of our position," and gives the software maker the "opportunity to tell the jury the whole story of how this technology was developed."

Microsoft could win the case if it proves that someone else invented the technology. If it fails, however, the verdict would stand and the company would still have to pay the plaintiffs, the University of California and Eolas Technologies Inc., more than $520 million.

Plaintiffs attorney Martin Lueck of the law firm Robins, Kaplan, Miller and Ciresi LLP in Minneapolis, was confident the company would fail.

"We don't believe that they're going to be able to do it," Lueck said. "They couldn't do it before the trial and they aren't going to be able to do it now.... What Microsoft gets is one last shot, or gasp, to prove that the patent is invalid, otherwise everything else stands."

In August 2003, a federal court jury sided with the UC and Eolas in their patent-infringement suit against Microsoft. Eolas founder Michael Doyle was granted the patent while he was an adjunct professor at the University of California, San Francisco, but the UC owns the technology.

US Patent No. 5,838,906 covers technology that enables a browser to call programs over the Internet to display streaming audio and video, advanced graphics and other content within a singe web page. The technology has become a standard within the language used to write web pages, called Hypertext Markup Language. The World Wide Web Consortium, which has asked federal officials to revoke the patent, controls the development of HTML.

Microsoft claims the technology covered by the patent was developed by Pei-yuan Wei and his colleagues at O'Reilly and Associates, who Microsoft says are the "true pioneers of this technology."

The company said the latest ruling would give it the opportunity "to present evidence that Eolas knowingly withheld information about Pei-Wei's invention to the patent office."

Lueck, however, argued that Doyle invented the technology and "no one had done it before, certainly not Pei-Wei."

"Microsoft can say what they want in the press, but they have to prove it in a courtroom," Lueck said.

The W3C, a leading Internet standard body, took the unusual step in October 2003 of asking the U.S. Patent and Trademark Office to revoke the patent that it claimed threatened "substantial economic and technical damage" to the web. It was the first time the W3C had asked that a patent be revoked.

In its letter to USPTO, the W3C claimed the Eolas patent is invalid, because its ideas had previously been published as prior art, which is a legal term. Prior art was not considered in the Microsoft trial, nor when the patent was granted, the standards body said. Therefore, the patent should be invalidated.