CSS has never supported constants or variables directly. Whenever I'm writing code like this:

span.class1 {
  color: #377fb6;
}

div.class2 {
  border: solid 1px #377fb6; /* Repeated color */
}

I wonder why such a seemingly simple feature has never made it into the standard. What could be hard about implementing a scheme whereby we could avoid repetition, something like this:

$theme_color1: #377fb6;

span.class1 {
  color: $theme_color1;
}

div.class2 {
  border: solid 1px $theme_color1;
}

I know there are workarounds, like using a class for each color or generating CSS code from templates, but my question is: given that CSS is so rich and complex, why weren't CSS constants ever introduced?