Disallow units for zero lengths.

a { top: 0px; }
/**      ↑↑
 * This zero and this type of length unit */

Lengths refer to distance measurements. A length is a dimension, which is a number immediately followed by a unit identifier. However, for zero lengths the unit identifier is optional. The length units are: em, ex, ch, vw, vh, cm, mm, in, pt, pc, px, rem, vmin, and vmax.



The following patterns are considered violations:

a { top: 0px }
a { top: 0.000em }

The following patterns are not considered violations:

a { top: 0 } /* no unit */
a { transition-delay: 0s; } /* dimension */
a { top: 2in; }
a { top: 1.001vh }