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<html><head><style> #myDIV { width: 300px; height: 200px; background-color: lightgreen; border-block-end-style: solid; border-block-end-width: 1px; animation: mymove 5s infinite;}@keyframes mymove { 50% {border-block-end-width: 25px;}}</style></head><body><h1>Animation of border-block-end-width</h1><p>Gradually change the border-block-end-width property from 1px to 25px, then back:<p><div id="myDIV"></div></body></html>