“Edge Highlight” (EHLT in CSS) is an axis found in some variable fonts that can be used to control the thickness of the highlighted part of edges in three-dimensional letterforms. Values are in thousandths of an em (at the current font size). For example, setting Extrusion Depth to 500 will result in a 50%-of-the-em extrusion, which would be 20 px if the font size is set to 40 px.

The Google Fonts CSS v2 API defines the axis as:

Default:	Min:	Max:	Step:
12	0	1000	1

Note that the default value is expected to differ per family, rather than be universally set for any implementation of this axis.

The axis was first used in the Nabla font color font, which uses isometric perspective to achieve its three-dimensional look. The highlighted edge, at its maximum setting, creates greater contrast between the face of the letterform and its extrusion. And, like all components of a color font, can be recolored from its white default.