常见的伪类（pseudo-classes）和伪元素（pseudo-elements）
伪类：:hover 鼠标放上去的效果
:actiive 点击之后效果
伪元素
：before
：after
<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <title>Document</title> <style type="text/css"> *{ margin: 0; padding: 0; } ul{ width:1000px; margin: 0 auto; counter-reset:li; } li{ list-style: none; } ul>li{ background-color: #ccc; margin-top: 10px; } ul>li:before{ content: counter(li); counter-increment:li; background-color: #333; border-radius:100%; padding: 0 5px; color: #fff; margin-right: 10px; } .test{ width: 200px; height: 200px; background: #000; color: #fff; } .test:before{ content: " "; color: red; width: 20px; height: 20px; background: #fff; float: left; } .test:after{ content:" "; color: green; width: 20px; height: 20px; background: #fff; float: right; } </style> </head> <body> <div class="test"> hhhkjkkj </div> <ul> <li>List item one</li> <li>The second item on the list</li> <li>Number three is a bit longer, with some lorem ipsum for good measure. Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.</li> <li>And here is number four.</li> <li>The fifth item on the list</li> <li>The sixth item on the list</li> </ul> </body> </html>
清浮动：
.clearfix{zoom:1;}
.clearfix:after {clear:both;content:'.';display:block; 0;height: 0;visibility:hidden;}
::selection 伪类选择器。
::selection 选择器匹配被用户选取的选取是部分，即用户鼠标去手动选择的部分。
只能向 ::selection 选择器应用少量 CSS 属性：color、background、cursor 以及 outline。