There appears to be different default interior padding values
controlled by the MARGINHEIGHT and MARGINWIDTH attributes.
In other words, if only one of these attributes is specified, the default for the
unspecified attribute will be different than the default for the attribute if BOTH
attributes are specified. [That looks confusing even to ME =)] It is possible
(though unverified) that in instances where only one value is present it is used
as the margin value for BOTH dimensions.
Simple IFRAME, no MARGINHEIGHT or MARGINWIDTH attributes applied:
Simple IFRAME, MARGINWIDTH=20; no MARGINHEIGHT specified:
Simple IFRAME, MARGINWIDTH=20, MARGINHEIGHT=20: