iOS 12.2 added support for the decimal keyboard using the inputmode spec but the decimal keyboard has no way to enter a - for a negative decimal value.
Created attachment 360115 [details] screenshot
<rdar://problem/47551201>
Unfortunately, it doesn't look like UIKit supports a number pad UIKeyboardType with a minus key (hyphen)... https://developer.apple.com/documentation/uikit/uikeyboardtype/uikeyboardtypenumbersandpunctuation?language=objc contains all the relevant keys, but it isn't a number pad :(
to test this on an iOS device. data:text/html,<input type="number" inputmode="decimal">