Netscape Layout Extensions - HTML and XHTML: The Definitive Guide - page 1119

Figure H-6. Change gutter widths with the <multicol>
gutter attribute (Netscape 4 only)
H.2.1.3. The width attribute
Normally, the <multicol> tag fills the current width of the current text
flow. To have your multiple columns occupy a thinner space, or to ex-
tend them beyond the visible window, use the width attribute to specify
the overall width of the <multicol> tag. The columns are resized so that
the columns plus the gutters fill the width you've specified. [*] The width
may be specified as an absolute number of pixels or as a percentage of
the width of the current text flow.