The proposal has been advanced by Netflix, Google and Microsoft and is under consideration by the World Wide Web Consortium. It has the backing of the BBC, among others.
FSF founder Richard Stallman said that while the W3C could not prevent companies grafting DRM onto HTML5, it could prevent the standardisation of DRM.
"...where the W3C stands is tremendously important for the battle to eliminate DRM," Stallman wrote in a post on the FSF website.
"If the DRM is implemented in the operating system, this could result in distribution of works that can't be played at all on a free operating system such as GNU/Linux."
Stallman said that among the arguments for standardising DRM, one was that a lack of standardisation would mean encapsulating more data and works in formats that could not be searched.
"I doubt that claim; video sites that use Flash have plenty of information in searchable HTML about the videos. Standardized (sic) DRM could just as easily harm searchability, if it leads to more use of DRM," he wrote
He said another argument was that the W3C needed to obey the wishes of these companies to remain "relevant". "...in other words, to be in a position to influence events," he wrote.
"However, it makes no sense to preserve that influence for some later decision that will be less important than this one. And is it even real influence? "Influence" maintained by obeying a master is more self-delusion than reality."