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<style> * { margin: 0; padding: 0; } .box { width: 200px; height: 200px;
position: relative; background: #000; margin: 100px auto; box-sizing: border-box
; } .box::before, .box::after { transform: all .3s; position: absolute; content:
""; top: -2px; right: -2px; bottom: -2px; left: -2px; background:
linear-gradient(45deg,rgba(247, 7, 7, 0.8),rgb(231, 216, 4),rgb(0, 255, 0));
z-index:-1; box-sizing: border-box; /* 可执行动画 */ /* animation: run 10s infinite
linear; */ } @keyframes run { 0%{ background: linear-gradient(45deg,rgba(238, 41
, 238, 0.8),transparent,rgb(0, 255, 0)); } 25%{ background: linear-gradient(
45deg,rgba(9, 243, 80, 0.8),transparent,rgb(34, 69, 224)); } 50%{ background:
linear-gradient(45deg,rgba(243, 122, 9, 0.8),transparent,rgb(241, 18, 18)); }
100%{ background: linear-gradient(45deg,rgba(238, 41, 238, 0.8),transparent,rgb(
0, 255, 0)); } } </style> </head> <body> <div class="box"></div> </body>
实现思路：
1.给需要添加边框的元素进行定位
2.通过给定位的元素添加伪元素::after ::before
3.通过改变伪元素的top,left,right,bottom值来实现边框
只保留下边框，只需将top,left,right都设置为0
.box::before, .box::after { transform: all .3s; position: absolute; content: ""
; top: 0; right: 0; bottom: -2px; left: 0; background: linear-gradient(45deg,
rgba(247, 7, 7, 0.8),rgb(231, 216, 4),rgb(0, 255, 0)); z-index:-1; box-sizing:
border-box; }