思路：
Q1:为什么下划线不用边框border 而使用背景色呢？
要实现动画效果，随着行盒的方向走
新知识点
线性渐变：linear-gradient
方法：linear-gradient(direction, color-stop1, color-stop2, ...)
详情见：https://blog.csdn.net/Bekind2010/article/details/130269260
Code
<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>下划线</title><style>.title span {background: linear-gradient(to right, #ff5f57, #45b6eb) no-repeat right bottom;background-size: 0px 2px;transition: background-size 1s;}.title span:hover {background: linear-gradient(to right, #ff5f57, #45b6eb) no-repeat left bottom;background-size: 100% 2px;}</style></head><body><h2 class="title"><span>党的路线、方针、政策“飞入寻常百姓家”</span></h2></body></html>效果图
