Despite his enthusiasm, Mr. Gundlach, like Mr. Joseph, told of wondering if Google would even allow such potentially self-harming extensions.

Image Michael Gundlach, an independent programmer, created an extension for Google's browser that hit No. 8 in popularity. Credit... Wingate Downs for The New York Times

Each read the rules, and when convinced that an ad-blocking extension wouldn’t be kicked off, began programming in earnest.

Mr. Joseph said he was “honestly a little surprised that they kept true to their word and allowed ad-blockers,” but then explained that in many ways, Google’s hands were tied. Programmers who make iPhone applications, whose work exists at the sufferance of Apple, he said, at least share in the profit. Google needs to give great latitude to programmers as a way of “keeping credibility with the people who make extensions.”

Both programmers said their tasks were much easier because they had borrowed from the Firefox add-on  whether its code or its carefully compiled affiliated list of Internet addresses where ads come from. The Chrome extensions work differently, though: ad blockers on Chrome cannot prevent the ads from arriving on a Web page, as is done on Firefox; instead they mask the ads after they arrive.

For now, Google is playing only on the fringes by tolerating ad-blocking programs on Chrome, despite the implied threat to its livelihood. Chrome is still a fringe browser  in the vastness of the Internet, 40 million users is still fringe  and ad-blocking browser additions are still a relatively fringe experience.

“Ad blockers are still used by a tiny proportion of the Internet population, and these aren’t the kind of people susceptible to ads anyway,” Wladimir Palant, who runs Adblock Plus on Firefox, wrote in an e-mail message. Adblock is the most downloaded add-on for Firefox and has more than seven million users.

Still, Google seems confident that Internet users will be able to distinguish between their ads and more aggressive display advertising.