HTML Attributes for <IFRAME ...>
MARGINWIDTH =integer
MARGINHEIGHT =integer

Usage Recommendation
Don't use it. thumbs down

MARGINWIDTH and MARGINHEIGHT work much the same as their frame counterparts. MARGINWIDTH and MARGINHEIGHT control the inner margins of the frame.

This Code Makes This 
<IFRAME 
 SRC="BeaverHabitat.html" 
 WIDTH=250 HEIGHT=250 
MARGINWIDTH=1
 MARGINHEIGHT=1
 >
<A HREF="BeaverHabitat.html">Stuff 
about the Beaver</A>
</IFRAME>

<IFRAME 
 SRC="BeaverHabitat.html" 
 WIDTH=250 HEIGHT=250 
MARGINWIDTH=30
 MARGINHEIGHT=30
 >
<A HREF="BeaverHabitat.html">Stuff 
about the Beaver</A>
</IFRAME>

These are good attributes to avoid. It's much better to let the browsers display the frame margins as they deem best.

<IFRAME SCROLLING="..."> >