Appears to not be supported in WebGL (or at least not my machine), though it's listed in the spec, e.g.:

uniform vec3 initialUniform = vec3(1.0, 0.0, 0.0);

This provides default values for uniforms which would be really nice for making more reusable shaders, but quite likely out of scope for gl-shader to shim. Maybe a wrapper package, or even a polyfill? Polyfill runs the risk of putting more burden on the package consumer, though also provides a leaner and more generic way of getting support across multiple frameworks/libraries.