MathJax v4 is the culmination of several years of work, and includes
significant new features, as well as improvements on existing
features. There are some potentially breaking changes, and in order
to not have these changes affect existing web sites, this release
increments the major version number so that sites using the
mathjax@3 URLs will be protected from these until they update to
version 4 explicitly. That is, this new release is an opt-in update.
Major New Features
Other Significant Changes
See also the MathJax v4.0 release notes for a list of the commits and bug fixes in this release.