width、height为auto或者100%的区别

一、规则

1. 某div不设置宽度，那么width默认为auto.

2. 某子元素div的width为100%（或者设置为等于父元素宽度的具体值，比如父元素width为100px，子元素width也设置为100px），则此子元素的宽度 = 父元素width值（不包括父元素边框，内边距）+子元素的边框、内边距宽度

4. 某个div的width不设置，或者设置为auto, 子元素的宽度会包含在父元素内，子元素边框、内边距都在不会叠加到子元素的长度上

二、实例

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        .main {
            background: red;
            height: 40px;
             700px;
            font-size: 30px;
            color: #fff;
            height: 80px;
            padding-left: 20px;
        }
        
        .right {
            background: green;
            height: 40px;
             auto;
            padding-left: 40px;
        }
    </style>
</head>

<body>
    <div class="main">
        <div class="right">right</div>
    </div>
</body>

</html>

　效果图：

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        .main {
            background: red;
            height: 40px;
             700px;
            font-size: 30px;
            color: #fff;
            height: 80px;
            padding-left: 20px;
        }
        
        .right {
            background: green;
            height: 40px;
             100%;
            padding-left: 40px;
        }
    </style>
</head>

<body>
    <div class="main">
        <div class="right">right</div>
    </div>
</body>

</html>

效果图：

如果right再加一个padding-right:40px，green区域要再加长40px

三、高度的设置类似于宽度，我们结合flexbox弹性布局看下效果：

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<style>
    .flex-container {
        display: flex;
        flex-direction: row;
        justify-content: flex-start;
        height: 200px;
    }
    /* 以下为辅助样式 */
    
    .flex-container {
        background-color: #F0f0f0;
    }
    
    .flex-container .flex-item {
        padding: 20px;
        height: 200px;
        background-color: #B1FF84;
    }
    
    .flex-container .flex-item:first-child {
        background-color: #F5DE25;
    }
    
    .flex-container .flex-item:last-child {
        background-color: #90D9F7;
    }
</style>

<body>

    <div class="flex-container">
        <div class="flex-item">1</div>
        <div class="flex-item">2</div>
    </div>
</body>

</html>

改为auto

想让文字垂直居中，设置line-height

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<style>
    .flex-container {
        display: flex;
        flex-direction: row;
        justify-content: flex-start;
        height: 200px;
        line-height: 200px;
    }
    /* 以下为辅助样式 */
    
    .flex-container {
        background-color: #F0f0f0;
    }
    
    .flex-container .flex-item {
        padding: 20px;
        height: auto;
        background-color: #B1FF84;
    }
    
    .flex-container .flex-item:first-child {
        background-color: #F5DE25;
    }
    
    .flex-container .flex-item:last-child {
        background-color: #90D9F7;
    }
</style>

<body>

    <div class="flex-container">
        <div class="flex-item">1</div>
        <div class="flex-item">2</div>
    </div>
</body>

</html>

发现并未居中，因为line-height的leading加在padding-top下面，所以居中位置会往下偏移20px

把上下的padding去掉后就可以垂直居中了