Our CSS font stack of monospace styled elements varies across web products without reason.
We should unify rules to one approach, best probably font-family: monospace, monospace as used in normalize.css.
Background of this CSS value is to tackle different cross-browser monospace font sizing quirks, see detailed explanation.
These two versions are exactly equivalent, I don't think we've been using any of the possible variants of this hack consistently. I grepped core and extensions and found the following number of usages:
Rule Usages font-family: monospace, 'Courier'; 18 font-family: monospace, monospace; 8 font-family: monospace, fixed; 2 font-family: monospace, 'Courer New'; 1
Also, @Edokter has introduced a MW core comparison at T35496: Misleading comment regarding preformatted font in commonElements.css