HTML element selector, class selector, ID selector are simple and practical
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>无标题文档</title>
<style>
a{
color:red;} /*元素选择器*/
.a1{
color:blue;} /*类选择器*/
#a2{
color:green;} /*ID选择器*/
</style>
</head>
<body>
<a>元素选择器红色文字</a>
<p class="a1">类选择器蓝色文字</p>
<p id="a2">ID选择器绿色文字</p>
</body>
</html>