SnuggleTeX is a free and open-source Java library for converting LaTeX to XML (usually XHTML + MathML).
I initially wrote it in 2008 and I'm no longer working on it, however I'm aiming to make sure it continues to work.
There are some self-contained examples of using SnuggleTeX included within the source code:
The SnuggleTeX source code includes a documentation & demos webapp.
I used to provide a public instance of this webapp, however I shut that down in early 2024.
If you're happy accessing the SnuggleTeX source code (and have Java & Maven installed), then you can run the webapp on your own computer by typing the following from the top of the SnuggleTeX source tree:
mvn install
cd snuggletex-webapp
mvn tomcat7:run
You should then be able to access the webapp via http://localhost:8080.