It's a problem in both the XML and RDF [1] worlds that the same representation gets used for everything. Part of the success of CSS is that it looks different from HTML, it could have been expressed with angle brackets and I think people would have had terrible trouble understanding where the HTML ends and the CSS begins.

[1] Oddly, SPARQL-in-Turtle isn't half bad to my eye, see https://spinrdf.org/sp.html