Update, 01 Jul after rereading parent: Agree: no, you cannot use both "serif" and "monospace" as the DEFAULT style (last) in a stylesheet, but there is nothing in the css spec to prevent you from including, by name, a monospace, sans-serif font, as the original code below was intended to illustrate
HOWEVER, membership in the "monospace" class has NOTHING WHATSOEVER to do with whether a font has or lacks serifs. Membership in "monospace" indicates ONLY that it is one of a class of fonts in which each character occupies the same amount of horizontal space.
</update #2>

/* I believe PM uses both <code> and .c   demerphq corrects me below*/

<style type="text/css">
<!--
code, .c {    
font-family: "unix_sans_serif_monospace choice here",
             and_another_if_desired,
             MAC_choice(s),
             windoze_choice(s),
             monospace;
}
-->
[download]

The w3c spec calls for the browser to use the first font it recognizes/has available. If neither specified font (pseudo-coded here as unix_font(s)) is available, try MAC, then windows... and only if no suitable font is avail, default to the system's default monospace. NB: This is NOT doing OS recognition -- it simply causes a compliant browser (today: most of 'em for the purpose of this discussion)to walk thru the list of faces specified by the designer (unless the user has overidden with a personal stylesheet). If a given font is available on a windows box with the name spec'ed in the first alt above, so be it: it's probably gonna' be acceptable.

Update #1 Reformatted code for easier readability, emended some language in previous paragraph for precision, 01 Jul