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<!DOCTYPE html> <html> <body> <h1>select required 属性</h1> <p>required 属性指定用户在提交表单之前需要选择一个值：</p> <form action="/action_page.php"> <label for="cars">选择一辆车：</label> <select name="cars" id="cars" required> <option value="">None</option> <option value="volvo">Volvo</option> <option value="saab">Saab</option> <option value="mercedes">Mercedes</option> <option value="audi">Audi</option> </select> <br><br> <input type="submit" value="提交"> </form> </body> </html>