The value can be:

• One of two keywords — none or normal
• <content-replacement> when replacing a DOM node. <content-replacement> is always an <image>.
• A <content-list> when replacing pseudo-elements and margin boxes. A content-list is a list of one or more anonymous inline boxes appearing in the order specified. Each <content-list> item is either contents or of type <string>, <image>, <counter>, <quote>, <target>, or <leader()>.
• An optional alternative text value of a <string> or <counter>, preceded by a slash (/).

The keywords and data types mentioned above are described in more detail below:

none

When applied to a pseudo-element, the pseudo-element is not generated. When applied to an element, the value has no effect.

normal

The default value. Computes to none for the ::before and ::after pseudo-elements. For other pseudo-elements, the content will be the initial (or normal) content expected for that ::marker, ::placeholder, or ::file-selector-button. For regular elements or page margin boxes, this computes to contents.

contents Experimental

Adds the contents of the element itself to the generated content value.

<string>

A sequence of characters enclosed in matching single or double quotes. Multiple string values will be concatenated (there is no concatenation operator in CSS).

<image>

An <image>, representing an image to display. This can be equal to a url(), image-set(), or <gradient> data type, or a part of the webpage itself, defined by the element() function.

<counter>

The <counter> value is a CSS counter, generally a number produced by computations defined by <counter-reset> and <counter-increment> properties. It can be displayed using either the counter() or counters() function.

counter()

The counter() function has two forms: 'counter(name)' or 'counter(name, style)'. The generated text is the value of the innermost counter of the given name in scope at the given pseudo-element. It is formatted in the specified <list-style-type> (decimal by default).

counters()

The counters() function also has two forms: 'counters(name, string)' or 'counters(name, string, style)'. The generated text is the value of all counters with the given name in scope at the given pseudo-element, from outermost to innermost, separated by the specified string. The counters are rendered in the indicated <list-style-type> (decimal by default).

<quote>

The <quote> data type includes language- and position-dependent keywords:

open-quote and close-quote

These values are replaced by the appropriate string from the quotes property.

no-open-quote and no-close-quote

Introduces no content, but increments (decrements) the level of nesting for quotes.

<target> Experimental

The <target> data type includes three target functions, <target-counter()>, <target-counters()>, and <target-text()> that create cross-references obtained from the target end of a link. See Formal syntax.

<leader()> Experimental

The <leader()> data type inclues a leader function: leader( <leader-type> ). This function accepts the keyword values dotted, solid, or space (equal to leader("."), leader("_"), and leader(" "), respectively), or a <string> as a parameter. When supported and used as a value for content, the leader-type provided will be inserted as a repeating pattern, visually connecting content across a horizontal line.

attr(x)

The attr(x) CSS function retrieves the value of an attribute of the selected element, or the pseudo-element's originating element. The value of the element's attribute x is an unparsed string representing the attribute name. If there is no attribute x, an empty string is returned. The case sensitivity of the attribute name parameter depends on the document language.

alternative text: / <string> | <counter>

Alternative text may be specified for an image or any <content-list> items, by appending a forward slash and then a string of text or a counter. The alternative text is intended for speech output by screen-readers, but may also be displayed in some browsers. Note that if the browser does not support alternative text, the content declaration will be considered invalid and will be ignored. The / <string> or / <counter> data types specify the "alt text" for the element.

content = 
  normal                                              |
  none                                                |
  [ <content-replacement> | <content-list> ] [ / [ <string> | <counter> ]+ ]?  |
  element( )                                          

<counter> = 
  <counter()>   |
  <counters()>  

<counter()> = 
  counter( <counter-name> , <counter-style>? )  

<counters()> = 
  counters( <counter-name> , <string> , <counter-style>? )  

<counter-style> = 
  <counter-style-name>  |
  <symbols()>           

<symbols()> = 
  symbols( <symbols-type>? [ <string> | <image> ]+ )  

<symbols-type> = 
  cyclic      |
  numeric     |
  alphabetic  |
  symbolic    |
  fixed       

<image> = 
  <url>       |
  <gradient>  

<url> = 
  url( <string> <url-modifier>* )  |
  src( <string> <url-modifier>* )  

The first five examples create generated content on pseudo-elements. The last three are examples of element replacement.

This example inserts generated text after the text of elements that have a particular class name. The text is colored red.

HTML

<h2>Paperback Best Sellers</h2>
<ol>
  <li>Political Thriller</li>
  <li class="new-entry">Halloween Stories</li>
  <li>My Biography</li>
  <li class="new-entry">Vampire Romance</li>
</ol>

CSS

.new-entry::after {
  content: " New!"; /* The leading space creates separation
                       between the DOM node's content and the generated content
                       being added. */
  color: red;
}

Result

This example inserts differently colored quotation marks around quotes.

HTML

<p>
  According to Sir Tim Berners-Lee,
  <q cite="http://www.w3.org/People/Berners-Lee/FAQ.html#Internet">
    I was lucky enough to invent the Web at the time when the Internet already
    existed - and had for a decade and a half.
  </q>
  We must understand that there is nothing fundamentally wrong with building on
  the contributions of others.
</p>
<p lang="fr-fr">
  Mais c'est Magritte qui a dit,
  <q lang="fr-fr"> Ceci n'est pas une pipe. </q>.
</p>

CSS

q {
  color: #00f;
}

q::before,
q::after {
  font-size: larger;
  color: #f00;
  background: #ccc;
}

q::before {
  content: open-quote;
}

q::after {
  content: close-quote;
}

Result

Note the type of quotes generated is based on the language. Browsers add open- and close-quotes before and after <q> elements by default, so the quotes in this example would appear without them being explicitly set. They could have been turned off by setting the respective content property values to no-open-quote and no-close-quote, or by setting them both to none. They can also be turned off by setting the quotes property to none instead.

This example combines a counter sandwiched between two <string>s prepended to all list items, creating a more detailed marker for list items (<li>) within unordered lists (<ol>).

HTML

<ol>
  <li>Dogs</li>
  <li>Cats</li>
  <li>
    Birds
    <ol>
      <li>Owls</li>
      <li>Ducks</li>
      <li>Flightless</li>
    </ol>
  <li>Marsupials</li>
  </li>
</ol>

CSS

ol {
  counter-reset: items;
  margin-left: 2em;
}
li {
  counter-increment: items;
}
li::marker {
  content: "item " counters(items, ".", numeric) ": ";
}

Result

The generated content on each list item's marker adds the text "item " as a prefix, including a space to separate the prefix from the counter, which is followed by ": ", a colon and an additional space. The counters() function defines a numeric items counter, in which the numbers of nested ordered lists have their numbers separated with a period (.) in most browsers.

This example is useful for print stylesheets. It uses an attribute selector to select every fully qualified secure link, adding the value of the href attribute after the link text as the content of the ::after pseudo-element.

HTML

<ul>
  <li><a href="https://mozilla.com">Mozilla</a></li>
  <li><a href="/">MDN</a></li>
  <li><a href="https://openwebdocs.org">OpenWebDocs</a></li>
</ul>

CSS

a[href^="https://"]::after
{
  content: " (URL: " attr(href) ")";
  color: darkgreen;
}

Result

The generated content is the value of the href attribute, prepended by "URL: ", with a space, all in parentheses.

This example inserts an image before all links. Two content values are provided. The later content value includes an image with alternative text that a screen reader can output as speech. If a browser does not support alternative text, this declaration will be considered invalid, with the previous content value displaying. This fallback content list includes an image and the message " - alt text is not supported - ".

HTML

<a href="https://www.mozilla.org/en-US/">Mozilla Home Page</a>

CSS

The CSS to show the image and set the alternative text is shown below. This also sets the font and color for the content. This will be used on browsers that display the alternative text and in browsers that don't support alternative text and show the the fallback content value.

a::before {
  /* fallback content */
  content: url("https://mozorg.cdn.mozilla.net/media/img/favicon.ico")
    " - alt text is not supported - ";
  /* content with alternative text */
  content: url("https://mozorg.cdn.mozilla.net/media/img/favicon.ico") /
    " MOZILLA: ";
  font:
    x-small Arial,
    sans-serif;
  color: gray;
}

Result

If using a screen reader, it should speak the word "MOZILLA" when it reaches the image. If supported (if the "alt text is not supported" is not showing), you can select the ::before pseudo-element with your developer tools selection tool, and view the accessible name in the accessibility panel.

In browsers that don't support the alternative text syntax the whole declaration containing the alt text is invalid. In this case, the previous content value will be used, showing the image and "alt text is not supported" text.

This example replaces a regular element! The element's contents are replaced with an SVG using the url() image function.

Pseudo-elements aren't rendered on replaced elements. As this element is replaced, any matching ::after or ::before are not generated or applied. To demonstrate this, we include an ::after declaration block, attempting to add the id as generated content. This pseudo-element will not be generated as the element is replaced.

HTML

<div id="replaced">This content is replaced!</div>

CSS

#replaced {
  content: url("mdn.svg");
}

/* will not show if element replacement is supported */
div::after {
  content: " (" attr(id) ")";
}

Result

When generating content on regular elements (rather than just on pseudo-elements), the entire element is replaced. This means that ::before and ::after pseudo-elements are not generated.

This example demonstrates how an element's contents can be replaced by any type of <image>, in this case, a CSS gradient. The element's contents are replaced with a linear-gradient(). With @supports, we provide alt text support and a repeating-linear-gradient() for browsers that support alt text with element content replacement.

HTML

<div id="replaced">I disappear</div>

CSS

div {
  border: 1px solid;
  background-color: #ccc;
  min-height: 100px;
  min-width: 100px;
}

#replaced {
  content: linear-gradient(#639f, #c96a);
}

@supports (content: linear-gradient(#000, #fff) / "alt text") {
  #replaced {
    content: repeating-linear-gradient(blue 0, orange 10%) /
      "Gradients and alt text are supported";
  }
}

Result

Check the browser compatibility chart. All browsers support gradients and all browsers support replacing elements with images, but not all browsers support gradients as a content value and not all browsers support alt text on replacements. If the browser displays a box with no gradient, replacing elements is supported, but gradients are not supported as a content replacement value. If the element is replaced with a striped gradient, the browser supports both.

This example replaces an element's content with a image-set(). If the users display has normal resolution the 1x.png will be displayed screens with a higher resolution will display the 2x.png image.

HTML

<div id="replaced">Mozilla</div>

CSS

#replaced {
  content: image-set(
    "1x.png" 1x,
    "2x.png" 2x
  );
}

Result

CSS-generated content is not included in the DOM. Because of this, it will not be represented in the accessibility tree and certain assistive technology/browser combinations will not announce it. If the content conveys information that is critical to understanding the page's purpose, it is better to include it in the main document.

If inserted content is not decorative, check that the information is provided to assistive technologies and is also available when CSS is turned off.

• Accessibility support for CSS generated content – Tink (2015)
• WCAG, Guideline 1.3: Create content that can be presented in different ways
• Understanding Success Criterion 1.3.1 | W3C Understanding WCAG 2.0
• Failure of Success Criterion 1.3.1: inserting non-decorative generated content Techniques for WCAG 2.0

• CSS generated content module
• Replaced elements
• ::after
• ::before
• ::marker
• contain
• quotes
• <gradient>
• image-set() function
• url() function