If you flip the order in which those styles are declared it will work (actually should work either way; I thought a specified class override a html targeted style while in the same context).

A class denotes something special. You don't need to repeat yourself here. The best way to eliminate side affects is by doing explicit targeting so your CSS only does what you say it can and nothing else.

html > body > div.header > h1 { color: #FFFFFF; }