前端小知识--为什么你写的height:100%不起作用？

为什么你写的height:100%不起作用？

这个知识不算冷门的，但是用的时候可能还是会有些懵逼，不能生效时搜一搜就能找到答案了，但是你真的懂了吗？为什么想要设置一个全屏元素的时候，高度不受%的控制？

1.百分比宽高的设定

按照w3c中的width和height属性，可以明确%设定宽高是根据父元素的宽高来的：
http://www.w3school.com.cn/cs...
http://www.w3school.com.cn/cs...

2.width:100%;

我们写下这样一段代码，随意设置一个背景色便于观察元素

<body>
    <div style="width:100%;height:100%;background-color:blueviolet;">
    width:100%;height:100%;
    </div>
</body>
//宽100%，我们现在看到的高是属于font-size的，而不是100%；

<body>
    <div style="width:100%;height:200px;background-color:blueviolet;">
    width:100%;height:200px;
    </div>
</body>
//效果如下

可以看到基本上宽的100%很容易就实现的，但是这里的height却不能设置成%比的（该元素会消失看不见），这是为什么呢？

3.浏览器是如何计算高度和宽度的

Web浏览器在计算有效宽度时会考虑浏览器窗口的打开宽度。如果你不给宽度设定任何缺省值，那浏览器会自动将页面内容平铺填满整个横向宽度。即我们不设置宽，会自动填满整个横向宽度，如下：

<div style="height:100%;">height:100%;</div>

但是高度的计算方式完全不一样。事实上，浏览器根本就不计算内容的高度，除非内容超出了视窗范围(导致滚动条出现)。或者你给整个页面设置一个绝对高度。否则，浏览器就会简单的让内容往下堆砌，页面的高度根本就无需考虑。
因为页面并没有缺省的高度值，所以，当你让一个元素的高度设定为百分比高度时，无法根据获取父元素的高度，也就无法计算自己的高度。
即父元素的高度只是一个缺省值：height: auto;我们设置height：100%时，是要求浏览器根据这样一个缺省值来计算百分比高度时，只能得到undefined的结果。也就是一个null值，浏览器不会对这个值有任何的反应。
各个浏览器对于宽高的解析也不相同，大家可以自己搜索一下。
参考：http://www.webhek.com/post/cs...

4.如何解决

现在你知道了吧，%是一个相对父元素计算得来的高度，要想使他有效，我们需要设置父元素的height;
要特别注意的一点是，在<body>之中的元素的父元素并不仅仅只是<body>，还包括了<html>。
所以我们要同时设置这两者的height，只设置其中一个是不行的：

        html,body{
            height: 100%;
            margin: 0;
            padding: 0;
        }

5.关于line-height居中的一点误解？

Document

height:100%;

" title="" data-original-title="复制">

<!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>
html,body{
height: 100%;
margin: 0;
padding: 0;
}
div {
color: white;
text-align: center;
font-size: 30px;
line-height: 100%;
background-color: blueviolet;
}
</style>
</head>

<body>

<div style=“height:100%;”>height:100%;</div>

</body>

</html>

全部代码如上，可以看到设置了line-height为100%没有居中，这是为什么呢，因为这时候的%是相对于字体尺寸的？所以直接作用于没有绝对高度的元素是不行的。

line-height属性说明：http://www.w3school.com.cn/cs...
这时候要想居中，可以如下，做一个div嵌套，一个负责高度，一个负责居中，虽然感觉并不会这样用到，但是居中还是很灵验的~

Document

height:100%;

" title="" data-original-title="复制">

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

    </span></span></span></span></span><span class="hljs-selector-class"><span class="css"><span class="hljs-selector-class"><span class="css"><span class="hljs-selector-class"><span class="css"><span class="hljs-selector-class"><span class="css"><span class="hljs-selector-class"><span class="css"><span class="hljs-selector-class">.div1</span></span></span></span></span></span></span></span></span></span></span><span class="css"><span class="css"><span class="css"><span class="css"><span class="css"> {
        </span></span></span></span></span><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute">background-color</span></span></span></span></span></span></span></span></span></span></span><span class="css"><span class="css"><span class="css"><span class="css"><span class="css">: blueviolet;
        </span></span></span></span></span><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute">position</span></span></span></span></span></span></span></span></span></span></span><span class="css"><span class="css"><span class="css"><span class="css"><span class="css">: relative;
    }

    </span></span></span></span></span><span class="hljs-selector-class"><span class="css"><span class="hljs-selector-class"><span class="css"><span class="hljs-selector-class"><span class="css"><span class="hljs-selector-class"><span class="css"><span class="hljs-selector-class"><span class="css"><span class="hljs-selector-class">.div2</span></span></span></span></span></span></span></span></span></span></span><span class="css"><span class="css"><span class="css"><span class="css"><span class="css"> {
        </span></span></span></span></span><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute">font-size</span></span></span></span></span></span></span></span></span></span></span><span class="css"><span class="css"><span class="css"><span class="css"><span class="css">: </span></span></span></span></span><span class="hljs-number"><span class="css"><span class="hljs-number"><span class="css"><span class="hljs-number"><span class="css"><span class="hljs-number"><span class="css"><span class="hljs-number"><span class="css"><span class="hljs-number">30px</span></span></span></span></span></span></span></span></span></span></span><span class="css"><span class="css"><span class="css"><span class="css"><span class="css">;    
        </span></span></span></span></span><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute">color</span></span></span></span></span></span></span></span></span></span></span><span class="css"><span class="css"><span class="css"><span class="css"><span class="css">: white;
        </span></span></span></span></span><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute">text-align</span></span></span></span></span></span></span></span></span></span></span><span class="css"><span class="css"><span class="css"><span class="css"><span class="css">: center;                    
        </span></span></span></span></span><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute">width</span></span></span></span></span></span></span></span></span></span></span><span class="css"><span class="css"><span class="css"><span class="css"><span class="css">: </span></span></span></span></span><span class="hljs-number"><span class="css"><span class="hljs-number"><span class="css"><span class="hljs-number"><span class="css"><span class="hljs-number"><span class="css"><span class="hljs-number"><span class="css"><span class="hljs-number">400px</span></span></span></span></span></span></span></span></span></span></span><span class="css"><span class="css"><span class="css"><span class="css"><span class="css">;
        </span></span></span></span></span><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute">position</span></span></span></span></span></span></span></span></span></span></span><span class="css"><span class="css"><span class="css"><span class="css"><span class="css">: absolute;
        </span></span></span></span></span><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute">left</span></span></span></span></span></span></span></span></span></span></span><span class="css"><span class="css"><span class="css"><span class="css"><span class="css">: </span></span></span></span></span><span class="hljs-number"><span class="css"><span class="hljs-number"><span class="css"><span class="hljs-number"><span class="css"><span class="hljs-number"><span class="css"><span class="hljs-number"><span class="css"><span class="hljs-number">50%</span></span></span></span></span></span></span></span></span></span></span><span class="css"><span class="css"><span class="css"><span class="css"><span class="css">;
        </span></span></span></span></span><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute">top</span></span></span></span></span></span></span></span></span></span></span><span class="css"><span class="css"><span class="css"><span class="css"><span class="css">: </span></span></span></span></span><span class="hljs-number"><span class="css"><span class="hljs-number"><span class="css"><span class="hljs-number"><span class="css"><span class="hljs-number"><span class="css"><span class="hljs-number"><span class="css"><span class="hljs-number">50%</span></span></span></span></span></span></span></span></span></span></span><span class="css"><span class="css"><span class="css"><span class="css"><span class="css">;
        </span></span></span></span></span><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute"><span class="css"><span class="hljs-attribute">transform</span></span></span></span></span></span></span></span></span></span></span><span class="css"><span class="css"><span class="css"><span class="css"><span class="css">: </span></span></span></span></span><span class="hljs-built_in"><span class="css"><span class="hljs-built_in"><span class="css"><span class="hljs-built_in"><span class="css"><span class="hljs-built_in"><span class="css"><span class="hljs-built_in"><span class="css"><span class="hljs-built_in">translateX</span></span></span></span></span></span></span></span></span></span></span><span class="css"><span class="css"><span class="css"><span class="css"><span class="css">(-50%)  </span></span></span></span></span><span class="hljs-built_in"><span class="css"><span class="hljs-built_in"><span class="css"><span class="hljs-built_in"><span class="css"><span class="hljs-built_in"><span class="css"><span class="hljs-built_in"><span class="css"><span class="hljs-built_in">translateY</span></span></span></span></span></span></span></span></span></span></span><span class="css"><span class="css"><span class="css"><span class="css"><span class="css">(-50%);
    }
</span></span></span></span></span></span><span class="hljs-tag"><span class="hljs-tag"><span class="hljs-tag"><span class="hljs-tag"><span class="hljs-tag"><span class="hljs-tag">&lt;/</span></span></span></span></span><span class="hljs-name"><span class="hljs-tag"><span class="hljs-name"><span class="hljs-tag"><span class="hljs-name"><span class="hljs-tag"><span class="hljs-name"><span class="hljs-tag"><span class="hljs-name"><span class="hljs-tag"><span class="hljs-name">style</span></span></span></span></span></span></span></span></span></span></span><span class="hljs-tag"><span class="hljs-tag"><span class="hljs-tag"><span class="hljs-tag"><span class="hljs-tag">&gt;</span></span></span></span></span></span>

</head>

<body>

<div style=“height:100%;” class=“div1”>
<div class=“div2”>height:100%;</div>
</div>

</body>