Tapping the Enter button on mobile without entering anything into the Email address field results in an Enter button that’s stuck tapped (inverted color) and never becomes untapped (original color).
Chrome, iOS 11.4:
Thanks for reporting this. I have reproduced it and created a test case.
It seems that this is a bug in mobile Safari, as it also happens in my reduced test case: https://codepen.io/hidde/pen/gzeWby (see screenshot below). The button keeps its hover style after error, even though it is no longer being hovered.
Interesting! If you wish I can take this up to Apple, or you can, whatever your preference.
If you could do that, it would be great, feel free to use my test case (it should stay on that URL for the foreseeable future)