i found the solution. it's a facelet's problem (i think) with chrome as said here.
i must include this:
<f:view contentType="text/html">
...
</f:view.
in my <body>...</body> section.

now,
alert(document.getElementsByTagName("BODY")[0]);
return to me [object HTMLBodyElement] like firefox, and things work fine.

thank you for your help (well, that line of code helped me find the problem) and sorry for the annoyance.