Despite calls for the participation of sites such as Facebook, Twitter and other big names, the biggest participants are the online encyclopedia Wikipedia and the social-news website Reddit.
The situation shows that, while technology companies are concerned about the legislation, the House's Stop Online Piracy Act (SOPA) and the Senate's Protect Intellectual Property Act (PIPA), the companies are not prepared to sacrifice a day's worth of revenue and risk the ire of users for a protest whose impact on lawmakers is hard to gauge.
Wikipedia and Reddit will black out their pages so visitors will see only information about Stop Online Piracy Act (SOPA) and the Protect Intellectual Property Act (PIPA).
Of the biggest tech sites that have voiced opposition to the legislation, only Google is planning any type of change to its site tomorrow. It too will have information about the bills, although users will still be able to conduct Google searches.
"Like many businesses, entrepreneurs and Web users, we oppose these bills because there are smart, targeted ways to shut down foreign rogue websites without asking American companies to censor the Internet," said a Google spokeswoman. "So tomorrow we will be joining many other tech companies to highlight this issue on our US home page."
That solution allows Google to keep revenue attached to its searches, while still highlighting the issue.
Microblogging service Twitter also declined to participate, with chief executive Dick Costolo taking on critics of the decision on Twitter over the weekend.
"Closing a global business in reaction to single-issue national politics is foolish," he wrote.
Costolo followed up with a Tweet stating the company will continue to take an active role in opposing the bills.
"Watch this space," he tweeted.
That position of criticizing the bills, but sitting out the blackout is echoed by many big tech companies, including several who wrote to Congress in November to complain about the legislation, such as AOL Inc, eBay Inc, Mozilla and Zynga Inc.
"We are not adjusting the consumer experience on our properties tomorrow, but we will be helping to drive awareness of key issues around these bills to our users," said Tekedra Mawakana, senior vice president for public policy at AOL.
In November, a number of technology companies wrote to key lawmakers expressing opposition to the bill, including eBay, Facebook, Google, Twitter and Mozilla.
Still, the blackout had signed up thousands of participating sites by late Tuesday and succeeded on at least one level: attracting the attention of lawmakers and industry leaders backing the bills. They were quick to attack it.
"This publicity stunt does a disservice to its users by promoting fear instead of facts," said Lamar Smith, chairman of the House Judiciary Committee and a sponsor of SOPA. "Perhaps during the blackout, Internet users can look elsewhere for an accurate definition of online piracy."
Former senator Chris Dodd, who now chairs the Motion Picture Association of America, labeled the blackout a "gimmick" and called for its supporters to "stop the hyperbole and PR stunts and engage in meaningful efforts to combat piracy."
The legislation, designed to curb access and payments to overseas websites that traffic in stolen content or counterfeit groups, has been a major priority for entertainment companies, publishers, pharmaceutical firms and many industry groups. They maintain the proposed law is critical to curbing online piracy they say costs them billions of dollars a year.
Internet companies have furiously opposed the legislation and have ramped up their lobbying efforts in recent months, arguing it would undermine innovation and free speech rights and compromise the functioning of the Internet.
It was seemingly on the fast track for approval by Congress until the White House criticized aspects of it over the weekend.
(Reporting By Sarah McBride and Jasmin Melvin; editing by Andre Grenon)
Corrects second paragraph to show that Wikipedia is an online encyclopedia, not dictionary.