<!DOCTYPE html><html><head> <meta charset="utf-8"> <title>JS Bin</title><style type="text/css">ul{ list-style-type:none; padding:0px; margin:0px; border:1px solid red; width:200px; text-align:center;}li:hover{ color:white; animation:a 1s infinite; -webkit-animation: a 1s infinite;}@keyframes a{ 0% {background:rgba(39, 121, 99, 0.61);width:60%;} 25%{background:rgba(49, 151, 99, 0.7);width:70%;} 50%{background:rgba(59, 121, 109, 0.61);width:80%;} 100%{background:rgba(69, 141, 119, 0.61);width:100%;}} @-webkit-keyframes a{ 0% {background:rgba(39, 121, 99, 0.61);width:60%;} 25%{background:rgba(49, 151, 99, 0.7);width:70%;} 50%{background:rgba(59, 121, 109, 0.61);width:80%;} 100%{background:rgba(69, 141, 119, 0.61);width:100%;}}</style></head><body> <ul> <li>hello</li> <li>world</li> <li>thanks</li> </ul></body></html>我想要背景长度变化，而文字不移动，要怎么修改呢