Thank you @suzanne.soy and @soegaard2: you are right concerning the files katex.min.css, and katex.min.js, I replace the files of same name at my local katex folder, with the ones from the following links: https://cdn.jsdelivr.net/npm/katex@0.13.0/dist/katex.min.css https://cdn.jsdelivr.net/npm/katex@0.13.0/dist/katex.min.js and the "\tag{1}" worked fine, thank you very much.

It’s great to be able to reference the equations also in the HTML generated document. Even the function "\boxed" worked fine. Thank you again for your time and support with this apparently simple, but far reaching functionality. Best regards.