为什么当我添加CSS文件时，我的列表会删除“YouTube”条目?

main.xul

main.xul

<?xml version="1.0"?>
<?xml-stylesheet href="/go.html?url=main.css" type="text/css"?>

<window id="main" class="MainClass" title="MY TEST" width="640" height="480" xmlns="https://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
    <listbox id="mainList" flex="1" class="MainListbox">
        <listitem label="Twitter"/>
        <listitem label="YouTube" id="youtubeID" class="YoutubeClass" oncommand="document.getElementById('youtubeID').startYoutube()"/>
    </listbox>
</window>
<?xml version="1.0"?>
<?x