CSS Pseudo-Elements Module

a::after {
  content: " (" attr(href) ")";
}

.dead-link {
  text-decoration: line-through;
}

.dead-link::after {
  content: url("https://developer.mozilla.org/shared-assets/images/examples/warning.svg");
  display: inline-block;
  width: 12px;
  height: 12px;
}

<p>
  The sailfish is named for its sail-like dorsal fin and is widely considered
  the fastest fish in the ocean.
  <a href="https://en.wikipedia.org/wiki/Sailfish"
    >You can read more about it here</a
  >.
</p>

<p>
  The red lionfish is a predatory scorpionfish that lives on coral reefs of the
  Indo-Pacific Ocean and more recently in the western Atlantic.
  <a href="" class="dead-link">You can read more about it here</a>.
</p>

a {
  color: blue;
  text-decoration: none;
}

a::before {
  content: "🔗";
}

.local-link::before {
  content: url("https://developer.mozilla.org/shared-assets/images/examples/firefox-logo.svg");
  display: inline-block;
  width: 15px;
  height: 15px;
  margin-right: 5px;
}

<p>
  Learning resources for web developers can be found all over the internet. Try
  out
  <a href="https://web.dev/">web.dev</a>,
  <a href="https://www.w3schools.com/">w3schools.com</a> or our
  <a href="https://developer.mozilla.org/" class="local-link">MDN web docs</a>.
</p>

details[open]::details-content {
  color: dodgerblue;
  padding: 0.5em;
  border: thin solid grey;
}

<details open>
  <summary>Example summary</summary>
  <p>Lorem ipsum dolor sit amet consectetur adipisicing elit.</p>
  <p>
    Architecto cupiditate ea optio modi quas sequi, esse libero asperiores
    debitis eveniet commodi hic ad.
  </p>
</details>

p::first-letter {
  font-size: 1.5rem;
  font-weight: bold;
  color: brown;
}

<p>
  Scientists exploring the depths of Monterey Bay unexpectedly encountered a
  rare and unique species of dragonfish. This species is the rarest of its
  species.
</p>

<p>
  When Robison and a team of researchers discovered this fish, they were aboard
  a week-long expedition.
</p>

p::first-line {
  font-size: 1.2rem;
  font-weight: bold;
  text-decoration: underline;
}

<p>
  In warm ocean waters around the world, you may see a strange sight: A fish
  leaping from the water and soaring dozens of meters before returning to the
  ocean's depths. Early Mediterranean sailors thought these flying fish returned
  to the shore at night to sleep, and therefore called this family of marine
  fish Exocoetidae.
</p>

::grammar-error {
  text-decoration: underline red;
  color: red;
}

<p contenteditable spellcheck="true">
  My friends is coming to the party tonight.
</p>

#rainbow-text {
  font-family: monospace;
  font-size: 1.5rem;
}

::highlight(rainbow-color-1) {
  color: violet;
  text-decoration: underline;
}
::highlight(rainbow-color-2) {
  color: purple;
  text-decoration: underline;
}
::highlight(rainbow-color-3) {
  color: blue;
  text-decoration: underline;
}
::highlight(rainbow-color-4) {
  color: green;
  text-decoration: underline;
}
::highlight(rainbow-color-5) {
  color: yellow;
  text-decoration: underline;
}
::highlight(rainbow-color-6) {
  color: orange;
  text-decoration: underline;
}
::highlight(rainbow-color-7) {
  color: red;
  text-decoration: underline;
}

<p id="rainbow-text">CSS Custom Highlight API rainbow</p>

const textNode = document.getElementById("rainbow-text").firstChild;

if (!CSS.highlights) {
  textNode.textContent =
    "The CSS Custom Highlight API is not supported in this browser!";
}

// Create and register highlights for each color in the rainbow.
const highlights = [];
for (let i = 0; i < 7; i++) {
  // Create a new highlight for this color.
  const colorHighlight = new Highlight();
  highlights.push(colorHighlight);

  // Register this highlight under a custom name.
  CSS.highlights.set(`rainbow-color-${i + 1}`, colorHighlight);
}

// Iterate over the text, character by character.
for (let i = 0; i < textNode.textContent.length; i++) {
  // Create a new range just for this character.
  const range = new Range();
  range.setStart(textNode, i);
  range.setEnd(textNode, i + 1);

  // Add the range to the next available highlight,
  // looping back to the first one once we've reached the 7th.
  highlights[i % 7].add(range);
}

li::marker {
  content: "✝ ";
  font-size: 1.2em;
}

<p>Group known as Mercury Seven:</p>
<ul>
  <li>Malcolm Scott Carpenter</li>
  <li>Leroy Gordon (Gordo) Cooper Jr.</li>
  <li>John Herschel Glenn Jr.</li>
  <li>Virgil Ivan (Gus) Grissom</li>
  <li>Walter Marty (Wally) Schirra Jr.</li>
  <li>Alan Bartlett Shepard Jr.</li>
  <li>Donald Kent (Deke) Slayton</li>
</ul>

p::search-text {
  color: crimson;
  background-color: wheat;
}

<p>
  Using your browser's "Find in page" functionality, find a word or phrase that
  appears in this sentence, and note how, in supporting browsers, each result is
  highlighted using the specified custom styles.
</p>

p::selection {
  color: red;
  background-color: yellow;
}

<p>
  Select a fragment of this paragraph, to see how its appearance is affected.
</p>

::spelling-error {
  text-decoration: wavy red underline;
}

<p contenteditable spellcheck="true">
  My friends are coegdfgfddffbgning to the party tonight.
</p>