While there have been breaking releases to the parser and code generator that are bundled with this release, there was no code updates to the code in this package, meaning that technically this is not a breaking release. However, there were some major changes made which we will go through now.
The parser and code generator have been updated in two specific ways. First, extra handling was added to escaping html entities, which previously was not present and although infrequently occurring, was technically a bug. Now, if you use &, , <, and/or > (or their plaintext representations) within a text node, and &, and/or " (or their plaintext representations) within an attribute's content, they will be properly escaped. Previously this wasn't the case.
Additionally, a new node type has been added: doctype. This node type, as you might be able to guess, is exclusively for doctypes, which used to be represented by a text node, but this can no longer be the case due to the enhanced entity escaping described above.
For plugin authors, this only matters if you are injecting or modifying a doctype declaration within your plugin. If so, make sure to change the node type from text to doctype. For custom parser authors, the same change must be made. For everyone else, you should be able to upgrade without issue, but as always, be careful and make sure to test before releasing.