Це стара версія документу!
https://validator.w3.org/#validate_by_input
http://www.mathmlcentral.com/Tools/FromMathML.jsp
https://www-archive.mozilla.org/projects/mathml/demo/tester.html
https://fred-wang.github.io/MathFonts/
http://www.eyeasme.com/Joe/MathML/MathML_browser_test
http://www.dessci.com/en/reference/MathMLTestSuite/