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<!DOCTYPE html> <html> <body> <form action="/demo/htmldom/action_page.php"> 选择文件：<input type="file" id="myFile" name="usr_file" required> <input type="submit"> </form> <p>请点击“试一试”按钮，查看是否必须在提交表单前在文件上传字段中选取文件。</p> <button onclick="myFunction()">试一试</button> <p id="demo"></p> <script> function myFunction() { var x = document.getElementById("myFile").required; document.getElementById("demo").innerHTML = x; } </script> </body> </html>