For now, we support both v2 and v3, since they both seem to work. We'd prefer to only support one major version for ease of maintenance, but currently v3 is in the RC stage, and v2 doesn't work with the latest versions of Node.js, so this seems worthwhile until v3 graduates to a full release.

Closes #3795.

Closes #3807.

This is the beginning of support for form-associated custom elements.

This is an internal refactoring to improve consistency. In general, we prefer to only deal with impls inside the impl classes. ParentNode's querySelectorAll implementation was not following that rule, creating NodeList wrappers instead of impls.

Closes #3804.

Impacts from dependency updates:

* whatwg-url now includes URL.parse().
* parse5 fixed a small bug around noframes elements.
* nwsapi now supports some selectors better, especially around :has(), although it also has some regressions.
* cssstyle and rrweb-cssom improve our CSS parsing and serialization capabilities, including around layers, colors, calc(), and var()