爲什麼'beforepaste'事件不會在webkit中觸發？

beforecopy事件被觸發，但事件之前沒有觸發事件。這是爲什麼？爲什麼'beforepaste'事件不會在webkit中觸發？

<!DOCTYPE HTML> 
<html lang="en"> 
<head> 
<meta charset="UTF-8"> 
<style type="text/css">#editor{width:300px; height:300px; border: 1px solid black;}</style> 
</head> 
<body> 
<div id="editor" contentEditable="true">editor</div> 
<script type="text/javascript"> 
var elEditor = document.getElementById("editor"); 

elEditor.addEventListener('beforecopy', function(e){ 
    console.log('beforecopy'); 
    e.preventDefault(); 
    e.stopPropagation(); 
}); 

elEditor.addEventListener('copy', function(e){ 
    console.log('copy'); 
}); 

elEditor.addEventListener('beforepaste', function(e){ 
    console.log('beforepaste'); 
    e.preventDefault(); 
    e.stopPropagation(); 
}); 

elEditor.addEventListener('paste', function(e){ 
    console.log('paste') 
}); 
</script> 
</body> 
</html>