RENEE MONTAGNE, Host:

Here's a question. Have you ever surfed the Web and the Net, clicked on a Web site that turned out to be sexually explicit? Well, Congress is trying to help you avoid that.

It's considering legislation that would require commercial Web sites containing sexually explicit material to carry a warning label. That could make it easier for filtering software to block those pages. The bill could also force non- pornographic sites to make a tough choice - adopt a label, or risk a prison sentence.

NPR's Larry Abramson reports.

LARRY ABRAMSON: A number of studies indicate that young people frequently stumble across unwanted pornography on the Internet, graphic sexual images that they are not looking for. Montana Senator Conrad Burns says that's why he's attached this language to legislation moving through Congress.

CONRAD BURNS: IT requires, if you have sexually explicit material in your Web site, one way or the other, you have to put that on your homepage.

ABRAMSON: That warning label could come in the form of invisible meta text, data that tells filtering software this really is porn. The goal, according to Rachel Brand of the Justice Department, is to give parents more control.

RACHEL BRAND: The point is to allow parents to protect their children from exposure to pornography if they wish, without prohibiting any content from being posted on the Internet. So this proposal wouldn't affect the substance or the appearance of any Web site, but for anyone who chooses to use a filter on his or her computer, it would help ensure that the filters pick up pornographic sites.

ABRAMSON: Note that supporters say they're not trying to keep anything off the Internet. Nine years ago, the Supreme Court struck down a law intended to block salacious material from the Web. Congress tried again with the Child Online Protection Act, which would've made racy content available only to those who could prove they were adults. That law was challenged and is still in litigation.

Civil liberties groups blocked both of those laws from taking effect, and Leslie Harris, of the Center for Democracy and Technology says she'll fight this latest attempt as well.

LESLIE HARRIS: This is a label that would require everything from an artist selling online, to Victoria's Secret, to, you know, the most hard-core pornography, all to carry the same label.

ABRAMSON: Opponents of those earlier laws said that Internet filtering software would protect users from unwanted porn. They argued that strict prohibitions on graphic online content were unconstitutional.

Now the government wants to give a boost to the filtering approach, but Leslie Harris says the government's experiment with mandatory filtering for TV viewers should serve as a caution.

HARRIS: This reminds me of the V-Chip, where we passed a law about exactly what the technology would be and exactly how we would address blocking content in the television context. And there has been no innovation in that space since.

ABRAMSON: Harris says this new requirement would create a terrible dilemma for safe sex sites, or for those that offer material posted by other people, such as social networking sites or auction sites. They would have to decide whether to carry a warning label or face potential criminal penalties the law would set down.

Senator Conrad Burns has some advice for those sites.

BURNS: Well, I would say that you better watch what you put on your Web site, what you advertise.

ABRAMSON: A 2003 law requiring labeling of explicit commercial e-mail messages has led to numerous fines and suits against spammers. But applying that standard to the World Wide Web will revive the debate over censorship online.

Larry Abramson, NPR News, Washington.

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MONTAGNE: This is NPR News. Transcript provided by NPR, Copyright NPR.