How can I use CSS calc() with inherit?

I would like to use inherit with calc() like this:

#foo {
  animation-duration: 10s;
}
#foo > .bar {
  animation-duration: calc(inherit + 2s); /* =12s */
}

But it don't seem to works.

Is it browsers bug or specs?

The inherit keyword can only exist alone as a value in a property declaration. It cannot be used with anything else as a value, because it's a value in and of itself. This means it cannot be used with functions like calc() either. See CSS2.1 and css3-cascade.

So to put it simply, the spec doesn't allow using inherit that way.

With this in mind, in order to use an inherited property value as part of a child declaration, make a custom property which will be inherited instead:

#foo {
  --duration: 10s;
  animation-duration: var(--duration);
}
#foo > .bar {
  /* --duration is automatically inherited - no need for inherit keyword */
  animation-duration: calc(var(--duration) + 2s); /* =12s */
}