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CSS :required 选择器
trycss_sel_required
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<!DOCTYPE html> <html> <head> <style> input:required { background-color: yellow; } </style> </head> <body> <h1>Demo of the :required selector</h1> <p>An optional input element:<br><input></p> <p>A required input element:<br><input required></p> <p>The :required selector selects form elements with a "required" attribute.</p> </body> </html>