::before (:before)

/* Add a heart before links */
a::before {
  content: "♥";
}

<q>Some quotes</q>, he said, <q>are better than none.</q>

q::before {
  content: "«";
  color: blue;
}

q::after {
  content: "»";
  color: red;
}

<span class="ribbon">Notice where the orange box is.</span>

.ribbon {
  background-color: #5BC8F7;
}

.ribbon::before {
  content: "Look at this orange box.";
  background-color: #FFBA10;
  border-color: black;
  border-style: dotted;
}

<ul>
  <li>Buy milk</li>
  <li>Take the dog for a walk</li>
  <li>Exercise</li>
  <li>Write code</li>
  <li>Play music</li>
  <li>Relax</li>
</ul>

li {
  list-style-type: none;
  position: relative;
  margin: 2px;
  padding: 0.5em 0.5em 0.5em 2em;
  background: lightgrey;
  font-family: sans-serif;
}

li.done {
  background: #CCFF99;
}

li.done::before {
  content: '';
  position: absolute;
  border-color: #009933;
  border-style: solid;
  border-width: 0 0.3em 0.25em 0;
  height: 1em;
  top: 1.3em;
  left: 0.6em;
  margin-top: -1em;
  transform: rotate(45deg);
  width: 0.5em;
}

var list = document.querySelector('ul');
list.addEventListener('click', function(ev) {
  if (ev.target.tagName === 'LI') {
     ev.target.classList.toggle('done');
  }
}, false);

<ol>
  <li>Crack Eggs into bowl</li>
  <li>Add Milk</li>
  <li>Add Flour</li>
  <li aria-current='step'>Mix thoroughly into a smooth batter</li>
  <li>Pour a ladleful of batter onto a hot, greased, flat frying pan</li>
  <li>Fry until the top of the pancake loses its gloss</li>
  <li>Flip it over and fry for a couple more minutes</li>
  <li>serve with your favorite topping</li>
</ol>

li {
  padding:0.5em;
}

li[aria-current='step'] {
  font-weight:bold;
}

li[aria-current='step']::after {
  content: " \21E6"; /* Hexadecimal for Unicode Leftwards white arrow*/
  display: inline;
}