HtmlUnit has been using CyberNeko HTML parser (http://nekohtml.sourceforge.net/) for a long time. But since the development was discontinued around 2014, we started our own fork, which now has many improvements.

As of version 2.68.0, neko-htmlunit also uses its own fork of Xerces (https://github.com/apache/xerces2-j). This made it possible to remove many unneeded parts and dependencies to ensure e.g. compatibility with Android.