Why do the :before and :after pseudo-elements require a 'content' property?

问题

Given the following scenario, why does the :after selector require a content property to function?

.test {
    width: 20px;
    height: 20px;
    background: blue;
    position:relative;
}
			
.test:after {
    width: 20px;
    height: 20px;
    background: red;
    display: block;
    position: absolute;
    top: 0px;
    left: 20px;
}

<div class=\"test\"></div>

Notice how you do not see the pseudo element until you specify the content property:

.test {
    width: 20px;
    height: 20px;
    background: blue;
    position:relative;
}
			
.test:after {
    width: 20px;
    height: 20px;
    background: red;
    display: block;
    position: absolute;
    top: 0px;
    left: 20px;
    content:\"hi\";
}

<div class=\"test\"></div>

Why is this the intended functionality? You would think that the display block would force the element to show up. Oddly enough, you can actually see the styles inside web debuggers; however, they do not display on the page.

回答1:

Here are some references to various W3C specifications and drafts:

Selectors Level 3

The ':before' and ':after' pseudo-elements can be used to insert generated content before or after an element's content.

The :before and :after pseudo-elements

Authors specify the style and location of generated content with the :before and :after pseudo-elements. As their names indicate, the :before and :after pseudo-elements specify the location of content before and after an element's document tree content. The 'content' property, in conjunction with these pseudo-elements, specifies what is inserted.

The content attribute

Initial: none

This property is used with the :before and :after pseudo-elements to generate content in a document. Values have the following meanings:

none - The pseudo-element is not generated.

The styling applied to ::before and ::after pseudo-elements affects the display of the generated content. The content attribute is this generated content, and without it present, the default value of content: none is assumed, meaning there is nothing for the style to be applied to.

If you don't want to repeat content:''; multiple times, you can override this simply by globally styling all ::before and ::after pseudo-elements within your CSS (JSFiddle example):

::before, ::after {
    content:'';
}

回答2:

Based on your comments on others' answers, I believe your question is actually:

Why must the content property for pseudo-classes be set in the CSS, as opposed to the content of non-pseudo-classes, which may be set in either HTML or CSS?

The reason is that:

by definition, pseudo-classes are dynamically created for every single element specified by a page's HTML markup
all page elements, including pseudo-classes, must have a content property to be displayed.
HTML elements like  do as well, but you can set their content property quickly using markup (or with CSS declarations).
Howver, unlike non-pseudo-class-elements, pseudo-classes cannot be given values in the markup itself.
∴ Therefore, all pseudo-classes are invisible (their 'content' properties have no value) unless you tell them not to be (by giving them value with CSS declarations).

Take this simple page:

<body>
<p> </p>
</body>

We know this page will display nothing, because the  element has no text. A more accurate way to rephrase this, is that the  element's content property has no value.

We can easily change this, by setting the content property of the h1 element in the HTML markup:

<body>
<p>This sentence is the content of the p element.</p>
</body>

This will now display when loaded, because the content property of the  element has a value; that value is a string:

"This sentence is the content of the p element."

Alternatively, we can cause the  element to be displayed by setting the content property of the  element in the CSS:

p { content: "This sentence is the content of the p element set in the CSS."; }

These two ways of injecting the string into the  element are identical.

Now, consider doing the same thing with pseudo-classes:

HTML:
  <body>
      <p class="text-placeholder">P</p>
  </body>

CSS:
  p:before { content: "BEFORE... " ; }
  p:after { content: " ...and AFTER"; }

The result:

BEFORE...  P ...and AFTER

Finally, imagine how you would accomplish this example without using CSS. It's impossible, because there is no way to set the content of the pseudo-class in the HTML markup.

You might be creative and imagine something like this might work:

<p:before>BEFORE... </p>
<p> P </p>
<p:after> ...and AFTER</p>

But, it doesn't, because <p:before> and <p:after> are not HTML elements.

In conclusion:

Pseudo classes exist for every markup element.
They are invisible by default, because they are initialized with no content property.
You CANNOT set the content property for pseudo classes with HTML markup.
∴ Therefore, pseudo-elements' content property must be declared with CSS declarations, in order to be displayed.

回答3:

The reason you need a content: '' declaration for each ::before and/or ::after pseudo-element is because the initial value of content is normal, which computes to none on the ::before and ::after pseudo-elements. See the spec.

The reason the initial value of content isn't an empty string but a value that computes to none for the ::before and ::after pseudo-elements, is twofold:

Having empty inline content at the start and end of every element is rather silly. Remember that the original purpose of the ::before and ::after pseudo-elements is to insert generated content before and after the main content of an originating element. When there's no content to insert, creating an additional box just to insert nothing is pointless. So the none value is there to tell the browser not to bother with creating an additional box.

The practice of using empty ::before and ::after pseudo-elements to create additional boxes for the sole purpose of layout aesthetics is relatively new, and some purists might even go so far as to call it a hack for this reason.
Having empty inline content at the start and end of every element means that every (non-replaced) element — including html and body — would by default generate not one box, but up to three boxes (and more in the case of elements that already generate more than just the principal box, like elements with list styles). How many of the two extra boxes per element will you actually use? That's potentially tripling the cost of layout for very little gain.

Realistically, even in this decade, less than 10% of the elements on a page will ever need ::before and ::after pseudo-elements for layout.

And so these pseudo-elements are made opt-in — because making them opt-out is not only a waste of system resources, but just plain illogical given their original purpose. The performance reason is also why I do not recommend generating pseudo-elements for every element using ::before, ::after.

But then you might ask: why not have the display property default to none on ::before, ::after? Simple: because the initial value of display is not none; it is inline. Having inline compute to none on ::before, ::after is not an option because then you could never display them inline. Having the initial value of display be none on ::before, ::after is not an option because a property can only have one initial value. (This is why the initial value of content is always normal and it is simply defined to compute to none on ::before, ::after.)

回答4:

Until you add content: ..., the psuedo-element doesn't actually exist.

Setting other style properties is not enough to force the browser to create the element.

来源：https://stackoverflow.com/questions/17067918/why-do-the-before-and-after-pseudo-elements-require-a-content-property

标签

css

pseudo-element

css-content

Why do the :before and :after pseudo-elements require a &#39;content&#39; property?

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