[ghswa]

"No. The output might be beautiful, but the language is most certainly not. Awful to debug."

Absolutely agree but, as far as I'm aware, this "more modern alternative" does not exist.

[nas]

Lout is quite a bit saner but doesn't have much traction. It seems to be in "deep maintenance" mode now.

[andolanra]

There's Patoline[^1], which is still in a somewhat early state, and its language is quite LaTeX-ish.

[^1]: http://patoline.com/index.html

[vmarsy]

What about this ? http://dlmf.nist.gov/LaTeXML/

Having some XML output seems to make it easier to add CSS rules ?

[SeanLuke]

Combines the complexity of LaTeX with the hideousness of XML. Great.