This property is a strange and very specific Microsoft beast. Microsoft, beginning in Windows 2000, has the ability to turn off its system underlines for accelerator keys until the ALT key is pressed (then the accelerator underline hint is displayed.) This property should have no effect in previous Microsoft OS versions, and if this system capability is turned off (or "on", depending on how you look at it), it will also have no effect.
While this works for underlined elements (the HTML "U" element), it also seems to work for any element, although a different pseudo-underline color seems to be generated in such cases.
u { accelerator: true }
Test