In Safari 15.2, everything is rounded down. 1/2 = 0, 8/9 = 0.
Also, it's not possible to type decimal numbers and have them displayed, although it appears they are actually being used internally. Initially, the "." button is disabled. If you press 0, then the "." button is enabled, disappears when clicked (good up to here), but then clicking any of the numbers don't change the display. So, typing "0.9 + 0.9" will look like "0 + 0" and will equal 1 (which is 1.8, rounded down). Also, if you click enough numbers, the numbers are disabled (as usual). So, the displayed state isn't matching the internal state, it appears.
Also, it's not possible to type decimal numbers and have them displayed, although it appears they are actually being used internally. Initially, the "." button is disabled. If you press 0, then the "." button is enabled, disappears when clicked (good up to here), but then clicking any of the numbers don't change the display. So, typing "0.9 + 0.9" will look like "0 + 0" and will equal 1 (which is 1.8, rounded down). Also, if you click enough numbers, the numbers are disabled (as usual). So, the displayed state isn't matching the internal state, it appears.