Everything described here is considered “public”: this is what you can rely on. We will try to maintain backward-compatibility, and we really often do, but there is no hard promise.
Anything else should not be used outside of WeasyPrint itself. We reserve the right to change it or remove it at any point. Use it at your own risk, or have dependency to a specific WeasyPrint version.
Since version 43, WeasyPrint only provides major releases and does not follow semantic versioning. This choice may look odd, but it is close to what many browsers do, including Firefox and Chrome.
Even if each version does not break the API, each version does break the way documents are rendered, which is what really matters at the end. Providing minor versions would give the illusion that developers can just update WeasyPrint without checking that everything works.
Unfortunately, we have the same problem as the other browsers: when a new version is released, most of the user’s websites are rendered exactly the same, but a small part is not. And the only ways to know that, for web developers, are to read the changelog and to check that their pages are correctly rendered.
More about this choice can be found in issue #900.