KaTeX has an 'output' option and one of his settings is 'mathml'. You can produce clean MathML this way and even mixed HTML/MathML output (default and what you

KaTeX has an 'output' option and one of his settings is 'mathml'. You can produce clean MathML this way and even mixed HTML/MathML output (default and what you see in the OP's example) with MathML for accessibility. Problem with the pure approach isn't KaTeX but the browser support.