In an article published on the Web site this week, free software advocate Stallman says browsers run programs that are not free, and they are most often written in JavaScript. JavaScript once was used for minor frills in Web pages but it now is being used for major jobs, he said.

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"Most browsers have a way to turn off JavaScript entirely, but none of them can check for Javascript programs that are nontrivial and non-free. Even if you're aware of this issue, it would take you considerable trouble to identify and then block those programs," Stallman said. "However, even in the free software community, most users are not aware of this issue; the browsers' silence tends to conceal it."

JavaScript programs can be offered for free by distributing the source code under a free license, said Stallman. "But even if the program's source is available, there is no easy way to run your modified version instead of the original. Current free browsers do not offer a facility to run your own modified version instead of the one delivered in the page. The effect is comparable to tivoization, although not quite so hard to overcome," he said.

Tivoization is defined in wikipedia as a concept in which a system uses software offered under a "copyleft" license, which removes restrictions, but then uses hardware to prevent users from running modified versions. It came about as a result of an issue with Tivo digital recorders.