What does a JS polyfill have to do with CSS? Also, try that out in IE 11 without a polyfill.

This works in IE without a JavaScript polyfill (lol): <html> <body> <div style="width: 400px; height: 400px; background: red; display: flex; align-items: center; justify-content: center;"> <div>TESTING</div> </div> </body> </html>