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<!DOCTYPE html> <html> <head> <meta charset="utf-8"> <title>本教程(w3big.com)</title> <style> input:invalid { border:2px solid red; } </style> </head> <body> <h3> :invalid 选择器实例演示。</h3> <input type="email" value="supportEmail" /> <p>请输入合法 e-mail 地址,查看样式变化。</p> </body> </html>