xhtml script src=".." will show me nothing

in xsl i have:
<script language="javascript" src="js/popcalendar.js"></script>
xsl doesn't read this ofcourse and returns this to the explorer that reads 
x(d)html.
But if I do this it returns nothing(view source gives a perfect document).
If i exclude the tag the xhtml is shown (ofcourse I get a script error due, I 
call functions from the external page in the xhtml doc).

How come?