It doesn't have to do with hardware, it's just client-side implementation. I had to write code for multiple key presses before. The browser will normally trigger a keydown and keyup event whenever something is pressed and released. The trick is to keep an array of keyCodes representing current keydowns; during keydowns add to array and during keyups, remove from array.
It is hardware related. USB has a maximum rollover limitation of 6 keys + 4 modifiers. Though keyboards can have n-key rollovers (i.e., greater than 10 keys simultaneously), you have to use PS/2.
Do they require custom drivers? I see no reason you can't have all-key rollover with that, unless the OS refuses to do so.
Not that I'm saying you're wrong, I really don't know if normal driverless keyboard input is capable of this. I certainly hope so. I just see essentially all 10-key rollover (or more) keyboards using PS/2 and citing that it doesn't work through USB.
