I would like to style a class based on another class not existing in the ancestors.
div:not(.evil-class) .element {
background-color: green;
}<div class="evil-class">
<div class="element">An element within the evil class</div>
</div>
<div class="element">An element NOT in the evil class</div>
div:not(.evil-class) .element means "Something with the class element that is descended from a div which does not have the class evil-class"
Your element is not descended from any div, so div:not(.evil-class) doesn't match any of the ancestors (which are only <body> and <html> in this case).
There is currently no way to express "None of the ancestors have a specific class" in CSS.
Selectors Level 4 proposes allowing :not() to contain complex selectors so in the future you may be able to do something like:
.element:not(div.evil-class *) {
background-color: green;
}<div class="evil-class">
<div class="element">An element within the evil class</div>
</div>
<div class="element">An element NOT in the evil class</div>