::before (:before) - CSS: Cascading Style Sheets 编辑

In CSS, ::before creates a pseudo-element that is the first child of the selected element. It is often used to add cosmetic content to an element with the content property. It is inline by default.

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

Note: The pseudo-elements generated by ::before and ::after are contained by the element's formatting box, and thus don't apply to replaced elements such as <img>, or to <br> elements.

Syntax

/* CSS3 syntax */
::before

/* CSS2 syntax */
:before

Note: CSS3 introduced the ::before notation (with two colons) to distinguish pseudo-classes from pseudo-elements. Browsers also accept :before, introduced in CSS2.

Examples

Adding quotation marks

One simple example of using ::before pseudo-elements is to provide quotation marks. Here we use both ::before and ::after to insert quotation characters.

HTML

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

CSS

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

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

Result

Decorative example

We can style text or images in the content property almost any way we want.

HTML

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

CSS

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

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

Result

To-do list

In this example we will create a simple to-do list using pseudo-elements. This method can often be used to add small touches to the UI and improve user experience.

HTML

<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>

CSS

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;
}

JavaScript

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

Here is the above code example running live. Note that there are no icons used, and the check-mark is actually the ::before that has been styled in CSS. Go ahead and get some stuff done.

Result

Special characters

As this is CSS; not HTML, you can not use markup entities in content values. If you need to use a special character, and can not enter it literally into your CSS content string, use a unicode escape sequence, consisting of a backslash followed by the hexadecimal unicode value.

HTML

<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>

CSS

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;
}

Result

Specifications

Browser compatibility

BCD tables only load in the browser

See also

• ::after
• content