Oh hai, Selectors level 4!

dabblet.css

/* Oh hai, Selectors level 4! */
/* https://drafts.csswg.org/selectors-4/  */
/* Try this at http://nightly.webkit.org/ */

div:before {
	content: attr(class);
}

.a.b:not(.c) {
	background: red;
	border-left: 10px solid;
}

/* Multiclass with the same specificity as just one class! */
:not(:not(.a), :not(.b), :not(.c)) {
	background: blue; /* Is overridden later with lime */
	border-left: 20px solid; /* Is applied anyway */
}

.a {
	background: lime
}

dabblet.html

<div class="a"></div>
<div class="a"></div>
<div class="a"></div>
<div class="a b"></div>
<div class="a b"></div>
<div class="a b"></div>
<div class="a b c"></div>
<div class="a b c" id="lol"></div>

dabblet.js

// alert('Hello world!');

settings.json

{"view":"split-vertical","fontsize":"100","seethrough":"","prefixfree":"1","page":"css"}