Nokogiri is an HTML, XML, SAX, and Reader parser in Ruby. Among Nokogiri's many features is the ability to search documents via XPath or CSS3 selectors. Nokogiri depends on libxml2 and libxslt to provide its functionality.
Nokogiri serves many HTML scraping needs.
Nokogiri is one of the most downloaded Rubygems, having been downloaded over 270 million times from the rubygems.org.
You can learn more about the Tidelift partnership with Nokogiri in this blog post.
Nokogiri parses and searches XML/HTML using native libraries (either C or Java, depending on your Ruby), which means it's fast and standards-compliant.
You can learn more about Nokogiri on the Nokogiri website.