Eric Lawrence has written a pretty scary post about browser security and malicious websites that hope to trick us:
When building applications that display untrusted content, security designers have a major problem— if an attacker has full control of a block of pixels, he can make those pixels look like anything he wants, including the UI of the application itself. He can then induce the user to undertake an unsafe action, and a user will be none-the-wiser.
And the problem is even worse on mobile:
Virtually all mobile operating systems suffer from the same issue– due to UI space constraints, there are no trustworthy pixels, allowing any application to spoof another application or the operating system itself
Source: The Line of Death