:not()

p:not(.irrelevant) {
  font-weight: bold;
}

p > strong,
p > b.important {
  color: crimson;
}

p > :not(strong, b.important) {
  color: darkmagenta;
}

<p>
  <b>Mars</b> is one of the most Earth-like planets. <b>Mars</b> day is almost
  the same as an Earth day, only <strong>37 minutes</strong> longer.
</p>

<p class="irrelevant">
  <b class="important">NASA</b>'s Jet <del>Momentum</del> Propulsion Laboratory
  is designing mission concepts to survive the <b>Venus</b> extreme temperatures
  and atmospheric pressure.
</p>

:not(<complex-selector-list>) {
  /* ... */
}

<p>I am a paragraph.</p>
<p class="fancy">I am so very fancy!</p>
<div>I am NOT a paragraph.</div>
<h2>
  <span class="foo">foo inside h2</span>
  <span class="bar">bar inside h2</span>
</h2>

.fancy {
  text-shadow: 2px 2px 3px gold;
}

/* <p> elements that don't have a class `.fancy` */
p:not(.fancy) {
  color: green;
}

/* Elements that are not <p> elements */
body :not(p) {
  text-decoration: underline;
}

/* Elements that are not <div>s or `.fancy` */
body :not(div):not(.fancy) {
  font-weight: bold;
}

/* Elements that are not <div>s or `.fancy` */
body :not(div, .fancy) {
  text-decoration: overline underline;
}

/* Elements inside an <h2> that aren't a <span> with a class of `.foo` */
h2 :not(span.foo) {
  color: red;
}

<p class="foo">I am a paragraph with .foo</p>
<p class="bar">I am a paragraph with .bar</p>
<div>I am a div without a class</div>
<div class="foo">I am a div with .foo</div>
<div class="bar">I am a div with .bar</div>
<div class="foo bar">I am a div with .foo and .bar</div>

/* Invalid rule, does nothing */
p:not(.foo, :invalid-pseudo-class) {
  color: red;
  font-style: italic;
}

/* Select all <p> elements without the `foo` class */
p:not(:is(.foo, :invalid-pseudo-class)) {
  color: green;
  border-top: dotted thin currentColor;
}

/* Select all <div> elements without the `foo` or the `bar` class */
div:not(.foo, .bar) {
  color: red;
  font-style: italic;
}

/* Select all <div> elements without the `foo` or the `bar` class */
div:not(:is(.foo, .bar)) {
  border-bottom: dotted thin currentColor;
}