Creating fake dialogue boxes and windows is nothing new, that has been around forever. Maybe using css to determine the os/browser and mimic each one’s style is the new concept. I’ll believe it when someone can show me a webpage that proves it live.
Luckily for everyone the spammers couldn't resist the temptation to throw in a few brightly coloured police tape GIFs, radiation symbols, typos, bold fonts in about 6 sizes and otherwise generally spoil the illusion.