CSS validator doesn't like variables

For example:

:root {
  --foo: 10px;
}

div {
  height: var(--foo);
}

gives:

at line 2Parse Error
--foo: 10px;
at line 3Parse Error
}
at line 6Value Error : height (nullvisudet.html#propdef-height) Parse Error
height: var(--foo);

The CSS validator at the W3 site gives the same thing, so maybe it's beyond the scope of this project, but perhaps there should be a mode that accepts variables?

leny / atom-w3c-validation

CSS validator doesn't like variables #43