Even if they expanded the system such that it were capable of automatically creating spans in the text to adjust from white to black depending upon the background color under specific letters, having the text switch from black to white and back again along with the background would look horrible and would be terrible for reading. But using high contrast outlines works fine on just about anything, which is why that is the go-to for all "meme" photos (Impact white with black outline). While I would suggest avoiding Impact unless you are actually trying to make a "meme" association, this sort of high contrast outlined text is still by far the best way to handle text over images that may themselves be high contrast.
Sadly, true text outline support is inexplicably very bad among browsers, at least as of the last time I checked, though in CSS3-capable browsers you can cheese outlines using textshadow.