HTMLIFrameElement - Web APIs
htmliframeelement.marginwidth is a domstring being the width of the frame margin.
... recommendation the following properties are now obsolete: scrolling, marginwidth, marginheight, longdesc, frameborder, and align.

Style System Overview - Archive of obsolete content
detail: !important declarations cause an extra rule object cssimportantrule to be created since they are in a separate part of the cascade other nsistylerule implementations nshtmlmappedattributes represents stylistic html attributes turned into a style rule (one instance per unique set of attributes) bodyrule handles marginwidth/marginheight mixes on body and on frame.

Mozilla Quirks Mode Behavior
frames in quirks mode marginwidth and marginheight on a frame are propagated to the contained body.

nsIDocShell
marginwidth long the value of the marginwidth attribute on the frame element, or negative if it was not set.

HTMLImageElement.hspace - Web APIs
syntax htmlimageelement.hspace = marginwidth; marginwidth = htmlimageelement.hspace; value an integer value specifying the width, in pixels, of the horizontal margin to apply to the left and right sides of the image.

<frame> - HTML: Hypertext Markup Language
marginwidth this attribute defines the width of the margin between frames.

<iframe>: The Inline Frame element - HTML: Hypertext Markup Language
marginwidth the amount of space in pixels between the frame's content and its left and right borders.