Use XML attribute paddingHorizontal on pre API 26 devies?
paddingHorizontal and paddingVertical is indeed introduced with API 26 (O). See the Android O update note for proof: https://developer.android.com/about/versions/oreo/android-8.0
To deal with this warning I changed the attributes for use the combination of paddingTop and paddingBottom, like this:
android:paddingTop="16dp" android:paddingBottom="16dp"
I think the support for android:paddingHorizontal is there even in API level 10 as seen in its website.