语法：linear-gradient(direction, color-stop 1, color-stop 2,……）
简单用法：background-image: linear-gradient(red, transparent);
增加角度，linear-gradient(45deg, red, transparent)
加个position：linear-gradient(45deg, red, transparent 45%)
加个colorlinear-gradient(45deg, red, transparent 45%,red)
linear-gradient(45deg, transparent 45%, red 55%, transparent 60%)
linear-gradient(135deg, transparent 45%, red 55%, transparent 60%)
把这两个线结合达到波浪线效果
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>Documenttitle>
<style>
.down{
position: relative;
}
.down:after{
content: '';
position: absolute;
bottom: -2px;
left: 5%;
width: 90%;
height: 2px;
background: -webkit-linear-gradient(315deg, transparent, transparent 45%, red 55%, transparent 55%, transparent 100%)
,-webkit-linear-gradient(45deg, transparent, transparent 45%, red 55%, transparent 55%, transparent 100%);
background-size: 4px 4px;
background-repeat: repeat-x;
}
style>
head>
<body>
<span class="down">燃情岁月span>
body>
html>
示例：
红色下划线
红色下划线