CSS selector for something not inside something else

You can use the Child Combinator Selector > to specify direct children:

.select-inside-this :not(.not-inside-this) .select-this,
.select-inside-this > .select-this {
    /* style here /*
}

This selects any .select-this element which is not a descendent of any .not-inside-this element and also selects .select-this elements which are direct children of .select-inside-this elements.

body > .select-inside-this :not(.not-inside-this) .select-this,
body > .select-inside-this > .select-this {
  color: red;
}

<div class="foo">
    <div class="select-inside-this">
        <div class="not-inside-this">
            <div class="one select-this">
                This should not be selected
            </div>
        </div>
    </div>
</div>
<div class="select-inside-this">
    <div class="two select-this">
        This should be selected
    </div>
</div>
<div class="three select-this">
    This should not be selected
</div>

:not(.not-inside-this) and *:not(.not-inside-this) with the * are equivalent; in the case of the former, the universal selector is implied. See the spec.

It is currently not possible to construct a CSS selector that matches elements that are not descendants of specific elements for the reasons given in the following questions:

• CSS negation pseudo-class :not() for parent/ancestor elements
• Is the CSS :not() selector supposed to work with distant descendants?

The selector

.select-inside-this :not(.not-inside-this) .select-this

matches .select-this elements that are descendants of some element that is not .not-inside-this, which in turn is a descendant of .select-inside-this. It does not match .select-this elements that are not descendants of .not-inside-this within .select-inside-this.

This means, first off, that your selector will incorrectly match the following:

<div class="select-inside-this">
    <div class="bar">
        <div class="not-inside-this">
            <div class="select-this"></div>
        </div>
    </div>
</div>

... because one of the ancestors of .select-this, .bar, is :not(.not-inside-this).

Additionally, this implies at least three levels of nesting (though it could be more). In your example, there are no other elements between .two.select-this and its containing .select-inside-this, so it will never match that element. This is why James Donnelly suggests adding .select-inside-this > .select-this to account for that particular case.

However it is still not possible to write a single complex selector using descendant combinators to match elements without a specific ancestor. The only way is to repeat the child combinator method with as many :not(.not-inside-this) as necessary, but this requires that you account for all possible cases. If you can't do that, then you're out of luck with CSS selectors.