General font families in stylesheets, i.e. serif, sans-serif, monospace; are not recognised by CoolReader. For instance, if a class is defined as having font-family: monospace, it will just be displayed with the default (usually serif) font. In order to actually display text as monospace you have to explicitly define a font (i.e. font-family: "Courier New").
This can be a problem on Android. The default internal CSS has the definition code:"Courier New",monospace. However, on Android there is no Courier New, and CoolReader fails to fallback to the default monospace font, so the text within <code> is displayed with the default serif font.