The U.S. Copyright Office has issued an interim regulation giving U.S. online-only works a copyright deposit exemption.
Here's an excerpt from the announcement:
The Copyright Office of the Library of Congress is adopting an interim regulation governing mandatory deposit of electronic works published in the United States and available only online. The regulation establishes that online-only works are exempt from mandatory deposit until a demand for deposit of copies or phonorecords of such works is issued by the Copyright Office. It also states that categories of online-only works subject to demand will first be identified in the regulations, and names electronic serials as the first such category for which demands will issue. In addition, the regulation sets forth the process for issuing and responding to a demand for deposit, amends the definition of a "complete copy" of a work for purposes of mandatory deposit of online-only works, and establishes new best edition criteria for electronic serials available only online.