As reported by @mrdoob in #695 (comment), it appears as though HDR environment maps are no longer HDR after being applied as skybox textures.

The canonical tester used by @mrdoob can be found here: http://model-viewer-tester.glitch.me/
It is based on <model-viewer> v0.5.1

I have forked the tester here: http://pool-egret.glitch.me/
The forked tester is based on <model-viewer> as deployed to our docs (e.g., built from master).

It's clear from lowering the exposure that HDR in the skybox appears to be lost.