In April we resolved that '0' is no longer supported as a valid value for angles.

filter was already shipping with support for hue-rotate(0) so Chrome added a use counter.

The results indicate that hue-rotate(0) is being used in 0.013% of page views.

To avoid breaking these pages, we could add an exception:
hue-rotate( <angle> )
->
hue-rotate( [ <angle> | <zero> ] )

This would be consistent with gradients and transform, which are already
exceptions explicitly specified to support unitless 0, for the same reason.