The sole criterion of a monospace font is that all glyphs have the same fixed width.... The effect is similar to a manual typewriter, and is often used to set samples of computer code.

I don't know that that fully backs the apparent intent of your observation re "no distinction between serif and sans-serif...." In fact, I found nothing that tells me that a monospace sans-serif is impossible/non-existant (but on the other hand I conceed, I have not found a monospace sans-serif yet, let alone one which is cross-platform, commonly available, and free.).

As for "more appropriately a user concern" I offer another view: that fonts are a design element and that good design makes the rendered matter easy for the visitor to read. (Update My reading suggests a general agreement today that sans-serif is -- on screen -- more readable, more readily comprehended, than serif, while the reverse may be true when presented on dead trees. </update>) Further, it's a non-intrusive design element, since the knowledgeable user can easily use a local stylesheet to override any element which obstructs comprehension.

For (perhaps excess) clarity: It seems to me that there is NO contradiction or inconsistency between my observation re the user's ability to spec a different face and my notion that the designer has an duty to make comprehension easy.

The point is, you cannot specify both font-family: monospace and font-family: sans-serif; they are mutually exclusive options thanks to the way the CSS specification has been written.

As for whether or not it's a design issue, I would say the flaw of the design is in the font itself, not in whether or not it has serifs. To wit, my monospace fonts make it clear that 1 is one but l is ell. This may vary from system to system, as the fonts included on a user's computer are not something over which a website has much control.

--
[ e d @ h a l l e y . c c ]

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

Before you think I've gone off completely, my background, before coding, was web design, and before that, I did commercial graphics, so I've had a bit of a background in typography.

but on the other hand I conceed, I have not found a monospace sans-serif yet, let alone one which is cross-platform, commonly available, and free.

There are monospaced sans-serif fonts. In fact, many monospaced fonts that have been around since before true-type days would not show serifs at smaller point sizes. But there are san-serif fixed width fonts --monaco being the one that first comes to mind, although it does have serifs on i, j, l, I, J and 1, but I believe that has more to do with the fact that I1l all look the same in a true san-serif.

I offer another view: that fonts are a design element and that good design makes the rendered matter easy for the visitor to read.

A good design does make things eaiser to read, but not everyone reads things the same way -- some fonts may be easier for one person to read than other. Font selection does more than just determine the legibility of a passage -- it also sets the tone. The same passage may take on different inflections if written in a blackletter, old style, transitional, modern, gothic, wood type, art nouveau, etc, etc. (I didn't even get to any of the 'fancy' type fonts)

My reading suggests a general agreement today that sans-serif is -- on screen -- more readable, more readily comprehended, than serif, while the reverse may be true when presented on dead trees.

Care to provide your sources? Unless things have changed dramatically in the last 10 years, when I dealt with this professionally, the issue didn't have to do with the medium -- it had to do with the length of the line being read -- longer lines can be read faster, with fewer errors, than san-serifed fonts, although people think san-serif fonts are prettier 1. The other thing to consider is that many fonts were specifically designed for screen reading, and so including a screen representation of a font designed for printing in a screen legibility test will skew the results 2

Most of the san-serif vs. serif arguments tend to be people arguing on discussion sites, rather than scientific research, from what I've seen. Font selection is a very, very personal thing to most graphic designers, which results in dumbasses only making information available as PDF, or using images for blocks of test. Even when there are tests, they tend to focus on point size as a measurement, rather than a font's aspect ratio.

I'm personally of the opinion that the content is more important than the presentation is most situations, and that the designer should make suggestions, but should understand that it's completely normal for someone to override their decisions, and that users should know how to override settings, should they ever come across design that they don't like.