I could easily read all the text on that demo page, even when looking at my monitor head-on. Turns out, the 'secret' text is coloured to blend in with a white

I could easily read all the text on that demo page, even when looking at my monitor head-on. Turns out, the 'secret' text is coloured to blend in with a white background, but the page doesn't actually enforce a white background; my light-grey background doesn't provide enough contrast to read a novel, but it brings out the 'secret' text nicely.