Detailed explanation of CSS3 and pseudo-elements to enable the underline to expand to both sides when the mouse is moved in

This article mainly introduces the relevant information on using css3+ pseudo-elements to achieve the effect of underlines expanding to both sides when the mouse is moved in. The article first gives a detailed introduction to facilitate everyone's understanding, and then provides a complete example code for everyone to refer to and learn. Friends who need it, please come and study together. Hope it helps.

Let’s take a look at the renderings first:

Implementation ideas:

Combine pseudo elements: before and: After is positioned in the middle of the bottom of the element and sets the width from 0 to 100% to achieve the goal.

Implementation method:

1. First define a block element (inline elements have no width and height) and modify the style to a rectangle with a light gray background color, and set relative positioning.

html code

<p></p>

css style

#underline{

    width: 200px;

    height: 50px;

    background: #ddd;

    margin: 20px;

    position: relative;

}

2. Set two pseudo elements: before and :after, and set them to have a background color of blue (that is, underline color), use absolute positioning to fix the two elements to the bottom middle position of #underline.

css style

#underline:before,

#underline:after{

    content: "";/*单引号双引号都可以，但必须是英文*/

    width: 0;

    height: 3px; /*下划线高度*/

    background: blue; /*下划线颜色*/

    position: absolute;

    top: 100%;

    left: 50%;

    transition: all .8s ; /*css动画效果，0.8秒完成*/

}

3. Set the mouse move-in effect.

css style

#underline:hover:before{/*动画效果是从中间向左延伸至50%的宽度*/

    left:0%; 

    width:50%;

}

#underline:hover:after{/*动画效果是从中间向右延伸至50%的宽度*/

    left: 50%; /*这句多余，主要是为了对照*/

    width: 50%;

}

Optimization

1. Although the purpose was achieved, two pseudo-elements were used, one extending 50% to the left and the other 50% to the right. , can only one extension to 100% achieve the goal?

css code

#underline:after{

    content: "";

    width: 0;

    height: 5px;

    background: blue;

    position: absolute;

    top: 100%;

    left: 50%;

    transition: all .8s;

}

#underline:hover:after{/*原理是left:50%变成0%的同时，宽度从0%变成100%*/

    left: 0%;

    width: 100%;

}

2. Only define the :after pseudo-element, and change it from 50% to the left with a width of 0 to 0% from the left with a width of 100%. It can be implemented, thereby achieving the purpose of streamlining the code, and adding :before to facilitate other operations.

Complete code

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    <p></p>

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