Hundreds of millions of Internet users will go online Wednesday and find their favorite websites … dark.

In a gigantic Internet protest, thousands of sites will shut down in opposition to the Stop Online Piracy Act (SOPA) and the Protect Intellectual Property Act (PIPA). The two media industry-backed bills, in the House and Senate, respectively, aim to curb online piracy.

“It’s a pitched battle between Hollywood and Silicon Valley,” says lawyer Scott L. Vernick, a partner at Fox Rothschild L.L.P. in Philadelphia, specializing in data security and privacy. “On the one hand, you have music companies and movie companies, and on the other the big Internet sites.”

Star of the protest is Web giant Wikipeda, which planned to shut all 3.8 million of its English-language pages from 12:01 a.m. to 11:59 p.m. Wednesday. Those pages will show nothing but a black message opposing SOPA. The move, discussed in the Wikipedia world since December, was confirmed Monday by founder Jimmy Wales after the idea received widespread support in the Wikipedia comunity.

Among the estimated 7,000 other sites involved are the popular social media site Reddit, as well as Mozilla, iHeartChaos, MoveOn.org, and BoingBoing. For cyberhacker group Anonymous, Wednesday will be a day of rest. Blogging platform WordPress offers a plug-in that gives bloggers a banner publicizing the protest. Even Google is weighing in. It won’t go dark, but it is placing a link on its home page to tell users about the issue.

Erik Martin, general manager of Reddit, said by phone from San Francisco, “We wanted to raise awareness” about the perceived threat posed by the bills. “We thought blacking out would focus people on making calls and telling friends about it.”

It’s a dramatic standoff between established industries – multimedia companies, publishers, recording companies, pharmaceuticals – and Internet sites that, while small in comparison, represent the present and future of the Web.

The battle has had its twists and turns. The film, music, and entertainment industries – including Disney, News Corp., the NFL, Time Warner, Viacom – spent more than $91 million last year lobbying for the bills. The biggest names on the Net – Google, AOL, Yahoo, Twitter – wrote a letter to key leaders in Congress warning against them.

This month, Sony and Nintendo, which had long supported the bills, withdrew support. A Senate vote on PIPA is scheduled for Tuesday, but that timing looks shaky, as its sponsor, U.S. Sen. Patrick Leahy (D., Vt.), searches for last-minute tweaks. SOPA had seemed headed for passage in the House. But on Saturday the White House, on its official blog, came out against “legislation that reduces freedom of expression, increases cybersecurity risk, or undermines the dynamic, innovative global Internet.” That sounded like a veto threat. SOPA was tabled until everyone could agree on its contents.

That left none other than Rupert Murdoch, chief executive officer of News Corp. and a big supporter of SOPA, grousing on Twitter: “So Obama has thrown in his lot with Silicon Valley paymasters who threaten all software creators with piracy, plain thievery.” Murdoch also lashed out at Google: “Piracy leader is Google who streams movies free, sells advts around them. No wonder pouring millions into lobbying.” A Google spokesperson called his tweets “nonsense.”

Exhilarating. King Kong vs. Godzilla. But what’s it all about, really?

It remains all too easy to steal images, video, audio, games, code, text, and other intellectual or actual property from Internet sites. Internet piracy around the world is said to be a $58 billion business, according to one industry group, although statistics (largely industry-provided) are murky and disputed. “Because film and music companies were slow to respond to piracy early on, unauthorized downloading of copyrighted material pretty much destroyed them,” says lawyer Vernick. “Now they’re trying to get that back.”

It’s a crime for U.S. websites to steal material or sell stolen property. The new bills go further in requiring sites such as Google, Yahoo, and YouTube “to be more responsible for the links on their sites,” Vernick says.

The laws effectively expand the government’s power to police links to foreign sites that steal or sell unauthorized material. U.S. sites could be forced to drop such links, drop ads associated with them, or shut down pages with such links. If the sites do not comply, they themselves could be blocked or shut down. Companies that discover stolen material or suspect links could move to have sites blocked or shut.

And that’s where the problem lies.

Sites such as Google have many thousands of links, put there by thousands of people. “People are worried that this law will shut down, or block users from, a whole site for one bad link,” Vernick says. “You don’t want a sledgehammer when you need a surgical knife.” It’s not clear whether it’s even possible for sites to police themselves, much less for the government to police all sites. “And an unenforceable law,” Vernick says, “is a lousy law.”

Martin of Reddit says, “They’re written extremely vaguely, extremely broadly, with a lot of power for both private companies and the U.S. attorney general, with not much oversight. And the language and definitions are just bad from a technical standpoint, so there’s a huge potential for abuse.”

That’s why Eric Schmidt, executive chairman of Google, told TheHill.com in December: “I understand the goal of what SOPA and PIPA are trying to do. Their goal is reasonable, their mechanism is terrible.”

Almost no one disagrees that tougher laws on piracy would be good. The question is whether PIPA and SOPA are those laws, and whether they have unintended consequences. Vernick asks a question many are asking on Black Wednesday: “Will the law undermine the characteristic vitality of the Web, the innovation, the job creation? I don’t know, but that’s what the White House is worried about.”