CSS: 在没有向某些元素添加内容之后

img and input are both replaced elements.

A replaced element is any element whose appearance and dimensions are defined by an external resource. Examples include images (<img> tags), plugins (<object> tags), and form elements (<button>, <textarea>, <input>, and <select> tags). All other elements types can be referred to as non-replaced elements.

:before and :after only work with non-replaced elements.

From the spec:

Note. This specification does not fully define the interaction of :before and :after with replaced elements (such as IMG in HTML). This will be defined in more detail in a future specification.

With span:before, span:after, the DOM looks like this:

<span><before></before>Content of span<after></after></span>

Evidently, that won't work with <img src="" />.

:before and :after are not required to work for replaced elements, and CSS specifications do not specify how they would work for them, and the concept of replaced element is somewhat vague.

The CSS 2.1 specification clearly suggests that they can work for replaced elements, just saying that it does not “fully define” how. This relates to the issue that a replaced element is expected to have its own visual rendering, which is not controlled by CSS, whereas the pseudo-elements should add something to the content of the element. The spec adds that this will be defined “in more detail” in a future specification, but this has not taken place so far.

Browser vendors just decided to avoid problems by not implementing these pseudo-elements for some elements at all.

It is not clear at all what “replaced element” means, and the meaning appears to have changed somewhat. It is often interpreted as meaning the same as empty element (an element with EMPTY declared content, i.e. an element that cannot have any content), but CSS 2.1 itself shows a sample style sheet with the selector br:before (though browsers have ignored this, implementing br their own way). It can be argued that more and more elements have moved into the scope of CSS rendering, at least in part. For example, an input element (incuding its font, colors, etc.) is largely controllable with CSS in modern browsers.

Current browsers (Firefox, IE, Chrome) do not seem to support the :after and :before pseudo-elements for empty elements other than hr. For hr, IE and Chrome place the generated content inside a bordered box, which is the implementation of hr; the content makes the box taller. Firefox places the content of both (!) pseudo-elements after the horizontal rule that is its implementation of hr. This variation illustrates the kinds of “interaction” problems that are referred to in CSS 2.1.

It is often claimed that these pseudo-elements cannot be used for empty elements since their HTML definitions do not allow any content. This is a category error. The syntax rules of a markup language do not restrict what you can do in CSS

To conclude, :after and :before are currently not usable for empty elements (except marginally for hr), but this is mainly due to implementations and may change in the future.

Elements that doesn't have closing tag are void elements and they can't display content inside them:

https://www.w3.org/TR/html5/syntax.html#void-elements

All Blink, Webkit and Quantum browsers allow you to create pseudo elements only on checkboxes but this is controversial since no spec allow this behavior.

Here an example: https://codepen.io/equinusocio/pen/BOBaEM/

input[type="checkbox"] {
appearance: none;
color: #000;
width: 42px;
height: 24px;
border: 1px solid currentColor;
border-radius: 100px;
cursor: pointer;
transition: all 100ms;
background-size: 30%;
outline: none;
position: relative;
box-sizing: border-box;
background-color: #eee;
transition: background-color 200ms;


&::before {
content: '';
position: absolute;
left: 2px;
top: 2px;
bottom: 2px;
height: 18px;
width: 18px;
border-radius: 50%;
background-color: currentColor;
will-change: transform;
transition: transform 200ms cubic-bezier(.01,.65,.23,1);
box-shadow: 0 1px 3px rgba(0, 0, 0, 0.3);
}


&:checked {
background-color: aquamarine;


&::before {
transform: translateX(100%);
}
}
}

I've spent several hours plucking out my hair only to find that some other css override content (or display:none) property of my selector.

For example, if the following code is written in some other place, before or after element will never show:

#id > child:before {
content: none!important;
}

<html>
<div id="id" class="class">
<child>
Before element is not showing
</child>


</div>
<style>
child:before {
content: 'before';
color: 'red';
}
</style>


</html>

Just find the css which is overwriting your style and spam stronger selectors and !important to make it work

#id>child:before {
content: none!important;
}

<html>
<div id="id" class="class">
<child>
Before element is <strong>showing</strong>
</child>


</div>
<style>
#id.class>child:before {
content: 'before'!important;
border: 1px solid red;
}
</style>


</html>

<img> is a replaced element and using :before or :after pseudo-elements on it works if the image fails to load and otherwise it does not work. If you intend to have a fallback in case of image load failure, the following css useful:

img{
position: relative;
}


img:after{
position: absolute;
content: "Any allowed type of content including a fallback image";
left: 0;
}

For a good example, please refer to https://css-tricks.com/7-practical-uses-for-the-before-and-after-pseudo-elements-in-css/