Comment by PaulHoule

Homiconicity can get you into trouble.

CSS and HTML have a dual relationship. You could certainly write CSS with an XML-like syntax but people would always get confused at whether they are looking at style or markup. Because HTML and CSS look completely different you never have that problem.

XSLT shares the same problem with the RDF specs coming out at the same time that it hid the production rules/logical nature of the system, had it looked more like

  <someElement someAttributeattribute=$x/> -> <anotherElement>$x</anotherElement>

it could have been quite different. But production rules never really sold (got told that by the marketing chief of vendor at a hotel bar after a conference they sponsored) and it's an interesting question why. They can do all kind of neat things like manage asynchronous processes that happen over a long period of time (like having a loan officer approve a loan) but nobody ever tried to use them to deal with the async comm problem in Javascript as far as I can tell.