Do not break on CSS4 parent selector

[skerit]

The current CSS4 draft allows us to write selectors like this:

th! [data-sort-active=true] {
    background-color: #f1f1f1;
}

There are polyfills available for browsers that don't support this yet. (which is every single one of them, I guess)

However: LESS won't compile this. It throws an 'Unrecognised input' error.
Couldn't this just be ignored?

[lukeapage]

In the past we haven't supported css rules until they are implemented in a browser. This stops us implementing stuff that is then changed.

How is it polyfilled?

[seven-phases-max]

Closing as duplicate of #1292.