Comment by GiovanniP

TeXmacs can export formulae in MathJax format--there is a setting for that; I have been able to copy formulae to the clipboard (using right-click -> Copy to Clipboard, not by selecting) obtaining what seemed to me the correct LaTeX code. I do not know what is the standard for copying MathJax; I have see this thread of November 2019 (https://github.com/mathjax/MathJax/issues/2240) where someone states that " MathJax output can't be copied directly from the page in version 3", so perhaps the copying of the output with the right click menu is the standard now. TeXmacs can also export to MathML, I tested it and the output was not satisfactory there.