<!DOCTYPE html> <html lang="en" dir="ltr"> <head> <meta charset="utf-8"> <title></title> <style media="screen"> .callout{position: relative; width: 100px; height: 50px; background: #f00; } .callout::before{ content: ""; position: absolute; top: -.4em; left: 1em; padding: .35em; background: inherit; border: inherit; border-right:0; border-bottom:0; transform: rotate(45deg); } </style> </head> <body> <div class="callout"> Only </div> </body> </html>
原文：https://www.cnblogs.com/ron123/p/9568850.html