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<!DOCTYPE html> <html> <head> <style> #myDIV { width: 300px; height: 200px; border: 1px solid black; animation: mymove 5s infinite; } @keyframes mymove { 50% {border-top-width: 15px;} } </style> </head> <body> <h1>Animation of border-top-width</h1> <p>Gradually change the border-top-width property from 1px to 15px, then back:<p> <div id="myDIV"></div> </body> </html>