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