Someone that did not wanted to generate syntax errors in older browsers ?

It feels like it's a passive aggressive way of giving us the finger (ugly syntax) while giving in to the obvious need for CSS variables we've been asking for since time eternal. http://www.w3.org/People/Bos/CSS-variables

    data-*

is for attributes in HTML. It makes sense for CSS properties, syntactically speaking to map these two in terms of prefix-solutions.