The Khronos Unlit Test sample model README says that for the test model using this extension:

The visible surface ... should appear as a uniform color, so much so that the 3D effect is intentionally lost.

And offers the following as examples of good and bad render results:

Good	Bad

Here are examples of how <model-viewer> renders this model w/ various settings, all if which seem incorrect (possibly for different reasons):

model only	HDR background-image	experimental-pmrem

I also tested to see how Filament's gltf_viewer handles this case. It seems to handle the model correctly (presumably it supports the KHT_materials_unlit extension).

Filament gltf_viewer