If you're only using 1 PayPal button on a page, you can simply remove the form tags and submit using onClick="this.form.action='https://www.paypal.com/cgi-bin/webscr';this.form.submit();"
However, if you put more than one button on a page, this submits the last one ... rather than figuring out which button I was on, I used the email code instead of the button code generated by PayPal.
I'm using a bootstrap theme, so used this:
<a href="https://www.paypal.com/cgi-bin/webscr?cmd=_s-xclick&hosted_button_id=XXX" class="btn btn-primary">Buy Now</a>
(replace a href="https://www.paypal.com/cgi-bin/webscr?cmd=_s-xclick&hosted_button_id=XXX with your PP code). PayPal generates a unique button id for each product. Bootstrap styles the link into button with class="btn btn-primary"
I have 4 buttons side by side in a price table - you can see it here: http://leanseekers.com/Nutrition-Coac... sorry, not quite finished with site or theme, so not everything matches.
I tested this is IE, FF and Chrome - no problems, so far.