Nokogiri (鋸) is an HTML, XML, SAX, and Reader parser. Among Nokogiri's many features is the ability to search documents via XPath or CSS3 selectors. Mike Dalessio will be providing assurances for Nokogiri as part of the Tidelift Subscription.
One of the most widely used gems, Nokogiri is a crucial part of the Ruby ecosystem. With over 221 million downloads, Nokogiri is one of the 20 most downloaded gems on Rubygems.org, and the 390,000 repositories depending on it (as noted on libraries.io) reinforces that fact.
Nokogiri is used by many other well-known open source projects, including Rails, Bootstrap, Jekyll, and Sinatra.
You can learn more about Nokogiri by visiting the project site.