CSS selector for something not inside something else

Answer 1

A little bit late to the party, and it might not match your use case, but this is what I ended up doing:

HTML:

CSS:

.select-inside-this .select-this {
    background: blue;
}
.select-inside-this .not-inside-this .select-this {
    background: none;
}

The trick is to positively select the negative element and just undo the style.

It'll work for simple use cases, at the very least.