F24 - contrast

is F24 - http://www.w3.org/TR/2013/NOTE-WCAG20-TECHS-20130905/F24
a failure even if the actual contrast meets the minimum contrast
requirement ratio? In other words, if an author were to specify black text
color in css (and left the default background-color of say white) where the
ratio meets the success criterion de facto, would this still fail since no
bgcolor was specified in the css (because the user could run into problems
if he did choose a different default background color in his browser
settings)?

Thank you in advance for any feedback