depthFailAppearance
The Appearance used to shade this primitive when it fails the depth test. Each geometry instance is shaded with the same appearance. Some appearances, like PerInstanceColorAppearance allow giving each instance unique properties.
When using an appearance that requires a color attribute, like PerInstanceColorAppearance, add a depthFailColor per-instance attribute instead.
Requires the EXT_frag_depth WebGL extension to render properly. If the extension is not supported, there may be artifacts.