CSS calc() requires unit

Most CSS properties require a unit, for example padding: 10px. If you use 0 you can omit the unit, like padding: 0.

An exception is the calc() function, which always requires a unit, even for 0.

.example-class {
  padding: 0; /* works */

  height: calc(100vh - 0); /* does NOT work */
  height: calc(100vh - 0px); /* works */
}

That is mainly important if you use CSS custom properties (aka variables) with a default value of 0 inside calc():

.example-class {
  height: calc(100vh - var(--header-height, 0)); /* does NOT work */
  height: calc(100vh - var(--header-height, 0px)); /* works */
}