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<!DOCTYPE html> <html> <body> <textarea rows="4" cols="50">单击我（或使用 Tab 键）以确保我获得焦点。 然后，单击输入字段以确保它获得焦点。</textarea><br><br> 输入字段：<input type="text" onfocus="getRelatedElement(event)"> <p><strong>注意：</strong> IE8 及更早版本不支持 FocusEvent 对象的 relatedTarget 属性。</p> <script> function getRelatedElement(event) { alert("The related element that lost focus was: " + event.relatedTarget.tagName); } </script> </body> </html>