CSS选择器之必填项伪类:required，选填项伪类:optional示例
<!DOCTYPE HTML> <html> <head> <meta charset="utf-8"> <title>三十课 - CSS选择器之必填项伪类:required，选填项伪类:optional示例</title> <style> input { padding: 0.5em 1em; } :required { color: red; } :optional { color: blue; } </style> </head> <body> <input type="text" value="三十课" required /> <input type="text" value="程序员的笔记本" /> </body> </html>
运行结果
三十课