Using 0 as CSS value breaks the calc function if the two values are not pixels.

The only difference between a , a , and a is in the units applied. A like 1 becomes a if you add length-units (1px, 1em, etc) and a if you add time-units (1s, 1ms, etc).

Actually

For zero and only zero, we can use a when CSS expects a .

0 is not allowed in some functions such as rgb or HSL, because these expects degree or percentages.