This page let you test the rendering font-family declaration for monospaced fonts. TODO: add some references here :-)

Erwin Dokter had the following explanation on Bugzilla #33496:

By default, a (Windows) browser has it's default font-sizes set at 16px for serif and sans-serif, and 13px for monospace. Except in IE, where you cannot set any font-sizes... it uses 16px for all fonts.
Vector has a base font-size of 0.8em, and most browsers *correctly* scale down all fonts, including the monospace font, accordingly. So monospace is shown at 0.8 x 13px = 10px (which is perceived as 'too small'). But when you assign any font besides just "monospace", those browsers will no longer treat it as monospace and use 0.8 x 16px = 13px instead.

Below are various rendering:

font-family: monospace;
font-family: "Courier New";
font-family: Courier;
font-family: monospace, monospace;
font-family: monospace, "Courier New";
font-family: monospace, Courier;
font-family: monospace, Verdana;
font-family: monospace, DOESNOTEXISTREALLY;