I think I found the cause of the problem.
I had an include file which is an html file with too many pairs of style (<style>...</style> tags.
I place everything within one style open and close tag and it works fine now.
By the way, this problem occurred in IE but not in Chrome.
From reading a bit, it seems that it would be better to link to a css file instead of my include since the css file is cached.
Thanks,
Ofer