O/T but was it XML Schema specifically, or just XML DTDs that you needed to validate against? For the latter case, there's [1] (my proj), plus the option to rewrite XSDs into DTDs (under natural assumptions eg no local element redefinitions) without having to wrestle with Xerces and/or libxml (which hasn't formally completed XSD support anyway) and emscripten. You might also have luck by transpiling Xerces/Java to JavaScript using Google's j2cl+closure-compiler (which works very well if you can swallow bazel and/or isolate the actual j2cl Java app from its bazel wrapper).

[1]: http://sgmljs.net