Whose fault is it is British folks aren't the majority in W3C or even none of the British companies manufacture browser who implement the CSS specs. Hence, words are spelled like they are in America.
It has nothing to do with how the W3C is stacked. Us Australians and Brits use American spelling ('color', etc) when programming for consistency. We would rather our APIs are spelled incorrectly (setColor()) than inconsistently (sometimes setColor() and sometimes setColour()).