Gmail on iOS forces a minimum line-height

[M-J-Robbins]

If setting line-height to a value less than the current font-size gmail will change it to line-height:normal;

So line-height:1, line-height:1em, line-height:100% all work as expected

However line-height:0.9, line-height:0.9em, line-height:99%, will all get converted to line-height:normal;

Same works for px values.
So font-size:20px; line-height:19px won't work
But font-size:19px; line-height:19px will work