通过iframe的id
<iframe id=myiframe src="about:<input type=checkbox name=mycheckbox value=mycheckbox>"></iframe>
<input type=button onclick="alert(myiframe.mycheckbox.checked)">
<iframe id=myiframe src="about:<input type=checkbox name=mycheckbox value=mycheckbox>"></iframe>
<input type=button onclick="alert(myiframe.mycheckbox.checked)">
<p>
<input type=button value=改变状态 onclick="myiframe.mycheckbox.checked=!myiframe.mycheckbox.checked">
<input type=button onclick="alert(document.frames.myiframe.document.all.mycheckbox.checked)">