举例，给一个div动态设置宽度如下:
vh，是指css中相对长度单位，表示相对视口高度（Viewport Height），1vh = 1% * 视口高度。
视口比例长度定义了相对于视口的长度大小，这是文档的可见部分。
通俗点讲，vh就是当前屏幕可见高度的1%，也就是说 height:100vh == height:100%
<style type="text/css">li:nth-child(n+3){color:red}
</style>
<ul>
<li>第一行</li>
<li>第二行</li>
<li>第三行</li>
<li>第四行</li>
<li>第五行</li>
<li>第六行</li>
</ul>
这样写吧：<div class="meter">
<span style="width: 25%"></span>
</div>
.meter {
height: 20px /* Can be anything */
position: relative
background: #555
-moz-border-radius: 25px
-webkit-border-radius: 25px
border-radius: 25px
padding: 10px
box-shadow: inset 0 -1px 1px rgba(255,255,255,0.3)
}
.meter >span {
display: block
height: 100%
border-top-right-radius: 8px
border-bottom-right-radius: 8px
border-top-left-radius: 20px
border-bottom-left-radius: 20px
background-color: rgb(43,194,83)
background-image: linear-gradient(
center bottom,
rgb(43,194,83) 37%,
rgb(84,240,84) 69%
)
box-shadow:
inset 0 2px 9px rgba(255,255,255,0.3),
inset 0 -2px 6px rgba(0,0,0,0.4)
position: relative
overflow: hidden
}
.orange >span {
background-color: #f1a165
background-image: linear-gradient(to bottom, #f1a165, #f36d0a)
}
.red >span {
background-color: #f0a3a3
background-image: linear-gradient(to bottom, #f0a3a3, #f42323)
}
.meter >span:after {
content: ""
position: absolute
top: 0left: 0bottom: 0right: 0
background-image: linear-gradient(
-45deg,
rgba(255, 255, 255, .2) 25%,
transparent 25%,
transparent 50%,
rgba(255, 255, 255, .2) 50%,
rgba(255, 255, 255, .2) 75%,
transparent 75%,
transparent
)
z-index: 1
background-size: 50px 50px
animation: move 2s linear infinite
border-top-right-radius: 8px
border-bottom-right-radius: 8px
border-top-left-radius: 20px
border-bottom-left-radius: 20px
overflow: hidden
}
