“Link rot” is the citation of an internet page that later 
disappears or is revised. It is a problem generally, but also for appellate court opinions. Studies several years ago showed that a substantial percentage of links in U.S. Supreme Court opinions were rotten. (See here and here.) Adam Liptak then wrote in the New York Times that “[t]he modern Supreme Court opinion is increasingly built on sand.”
Some courts have responded by creating pages on their own websites that contain copies of the internet pages as they appeared when cited. The U.S. Supreme Court and the Ninth Circuit have them (more recently, the Ninth Circuit transitioned to putting the copies on individual internet case dockets).
The California Supreme Court has joined the crowd. The court now has a web page archive. Fittingly, the first case with citations in the archive is last week’s decision about Facebook and other internet-based social media.
The court’s archive includes this prefatory explanation:
Because some Internet uniform resource locators (URLs) included as citations in opinions issued by the Supreme Court of California may change over time or disappear altogether, this archive, as of May 24, 2018, preserves the content found at the URLs provided in these opinions, as it appeared at the time the opinions were filed.
[Note: all links in this post are accurate as of publication time only.]