Re: [w3ctag/design-reviews] WebXR DOM Overlay Module (#470)

> Yes, this is a technical limitation of additive see-through headsets such as HoloLens. The waveguide optics can only add light to the scene, they don't have any way to darken the perceived image, and therefore cannot display an opaque black. The result is basically equivalent to a `lighten` [blend mode](https://developer.mozilla.org/en-US/docs/Web/CSS/blend-mode) due to the way the hardware works.

I got the blend modes mixed up here, I think the additive mode more closely corresponds to the `screen` blend mode (_"like two images shone onto a projection screen"_), or `glBlendFunc(GL_ONE, GL_ONE)`.

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