An introduction to pseudo-elements in CSS and their differences from pseudo-classes

Pseudo-elements

We know that with the further improvement of CSS specifications, more and more new CSS pseudo-elements have been added, but in daily development, we commonly use The browser support situation that is more optimistic is before and after. But what we use in daily development are: after {content: ”;} to clear floats and add an element (take care of the use of a single colon in IE8 browser). But what are the possible values ​​​​of content?
1. String: content: “a string” - Note: Special characters must be encoded in unicode;
2. Picture: content: url(/path/to/benjamin.png) – The picture is inserted in its original size, Cannot be resized. Because the image supports gradients, you can use gradient effects on pseudo elements;
3. No characters: content: "" - This is more useful for clearing floats and setting background images. We can set the width and width of the background image. height, we can even use the background-size attribute to adjust the size of the background image;
4. Counter: content: counter(li)- Before :marker appears, it is more useful for setting the list serial number style; see the following code for details:

ol {   
    countercounter-reset: li;   
}   
ol>li {   
    position: relative;   
    padding-left: 2em;   
    line-height: 30px;   
    list-style: none;   
}   
ol>li:before {   
    position: absolute;   
    top: 8px;   
    left: 0;   
    height: 16px;   
    width: 16px;   
    line-height: 16px;   
    text-align: center;   
    content: counter(li);   
    countercounter-increment: li;   
    border-radius: 50%;   
    background-color: #ccc;   
    font-size: 12px;   
    color: #efefee;   
}

PS: We cannot set content: "c1a436a314ed609750bd7c7d319db4daBenjamin2e9b454fa8428549ca2e64dfac4625cd", it will not be parsed as HTML code snippets, but will be parsed as a string;
5. content: attr(attrName)
content can use the attr function to obtain the attribute value, which is especially convenient when used in pseudo classes. See the following code:

<style type="text/css">   
    .list li {   
        list-style: none;   
        margin-bottom: 20px;   
    }   
    .list li span {   
        vertical-align: middle;   
    }   
    .list li:before {   
        content: attr(data-index);   
        display: inline-block;   
        width: 20px;   
        height: 20px;   
        text-align: center;   
        color: #fff;   
        vertical-align: middle;           
        background-color: #f00;   
        border-radius: 50%;   
    }   
</style>   
<ul class="list">   
    <li data-index="1"><span>专注前端开发和用户体验</span></li>   
    <li data-index="2"><span>专注前端开发和用户体验</span></li>   
    <li data-index="3"><span>专注前端开发和用户体验</span></li>   
    <li data-index="4"><span>专注前端开发和用户体验</span></li>   
    <li data-index="5"><span>专注前端开发和用户体验</span></li>   
</ul>

Having said the previous words, let’s talk about the bugs encountered in IE:
Bug description: When using pseudo classes to implement "+"/"-" image switching, it is achieved by adding and removing the opened class, but in IE8 The effect is weird and cannot be rendered correctly. It is normal in other browsers:

.plus {   
    position: relative;   
    display: inline-block;   
    vertical-align: top;   
    width: 20px;   
    height: 20px;   
    margin-right: 24px;   
    border: 1px solid #fdaa47;   
    border-radius: 3px;   
    overflow: hidden;   
}   
/* 横向 */
.plus:before {   
    content: '';   
    position: absolute;   
    top: 10px;   
    left: 3px;   
    width: 14px;   
    height: 1px;   
    background-color: #fdaa47;   
    display: block;   
}   
/* 纵向 */
.plus:after {   
    display: block;   
    content: '';   
    width: 1px;   
    height: 14px;   
    background-color: #fdaa47;   
    position: absolute;   
    left: 10px;   
    top: 3px;   
}   
.opened:after {   
    top: -30px;   
}

When using addClass('opened') and removeClass('opened'), switch between addition and subtraction Time: The effect in IE8 browser is not as expected, and some styles cannot be covered. The current solution is as follows:

<p class="parent">   
    <i class="plus"></i>   
</p>   
<script type="text/javascript">   
$(&#39;.parent&#39;).on(&#39;click&#39;, function() {   
    var $i = $(this).find(&#39;i&#39;),   
        className = $i.attr(&#39;class&#39;);   
    className = /opened/.test(className) ? &#39;plus&#39; : className +&#39; opened&#39;;   
    $i.replaceWith(&#39;<i class="&#39;+ className +&#39;""></i>&#39;);   
});   
</script>

pseudo-class and Similarities and differences of pseudo-elements
1. W3C CSS 2.1 Selectors
There is no distinction between pseudo-classes and pseudo-elements. They all use a colon
For example,
pseudo-class: first-child,
Pseudo-element: first-line
PS: The specification clearly mentions the writing order of several pseudo-classes of a link:
Note that the A:hover must be placed after the A:link and A :visited rules, since otherwise the cascading rules will hide the 'color' property of the A:hover rule. Similarly, because A:active is placed after A:hover, the active color (lime) will apply when the user both activates and hovers over the A element.

2. CSS Selectors Level 3
This specification makes a distinction between pseudo-classes and pseudo-elements. Pseudo-classes use a single colon. Elements start with a double colon.
For example
Pseudo class: first-child
Pseudo elements::first-line, ::first-letter, ::before, ::after
CSS 3 is added on the basis of CSS2.1 There are many pseudo-classes: target, UI element status pseudo-classes: checked, etc., structural pseudo-classes: nth-child(), etc. For details, please refer to the specification.

3. CSS Selectors Level 4 draft
This draft adds many new pseudo-classes, such as those related to input control status, value status, and value verification. Pseudo-classes, tree-structured pseudo-classes, grid-structured pseudo-classes, etc.

4. CSS Pseudo-Elements Module Level 4——W3C First Public Working Draft, 15 January 2015
Added some pseudo-elements, such as:
Selecting Highlighted Content: the ::selection, ::spelling-error, and ::grammar-error pseudo-elements,
Placeholder Input: the ::placeholder pseudo-element.

5. Common applications
Pseudo-class:
1) a link style
2) Interlaced color change
Pseudo-element:
1 ) The most common use of pseudo-element after is to clear floats,
.fix{*zoom:1;}
.fix:after,.fix::after{display: block; content: “clear”; height: 0 ; clear: both; overflow: hidden; visibility: hidden;}
2) letter-spacing+first-letter to hide button text
3) First line and first letter style

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