W3School TIY Editor
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<!DOCTYPE html> <html> <head> <meta name="viewport" content="width=device-width, initial-scale=1"> <style> .mystyle { width: 100%; padding: 25px; background-color: coral; color: white; font-size: 25px; box-sizing: border-box; } </style> </head> <body> <p>点击“试一试”按钮将“mystyle”类添加到 DIV 元素：</p> <button onclick="myFunction()">试一试</button> <div id="myDIV"> This is a DIV element. </div> <script> function myFunction() { var element = document.getElementById("myDIV"); element.classList.add("mystyle"); } </script> </body> </html>