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<!DOCTYPE html><html><head><style>div {width:4px;height:400px;background:red;position:relative;animation:rainbow_bar 4s infinite;animation-timing-function:linear;animation-direction:alternate;}@keyframes rainbow_bar {0%{background:red;left:0px;}25%{background:yellow;left:200px;}50%{background:blue;left:400px;}75%{background:green;left:600px;}100%{background:red;left:800px;}}</style></head><body><div></div></body></html>