Cannot read to / write from `Depth16Unorm` texture on WebGL backend

[teh-cmc]

Description

I have a program that creates and fills Depth16Unorm depth textures on the CPU, then reads that data (textureLoad) in a shader to render point clouds.

@group(1) @binding(1)
var depth_texture: texture_2d<f32>;

let norm_linear_depth = textureLoad(depth_texture, texcoords, 0).x;

On native, this works without issue, as the spec suggests.

When using the WebGL backend, on the other hand, my shader just... doesn't work. I cannot see any relevant logs in the browser console, and trying to enforce a fragment color output does nothing, so I assume the shader crashes right at the textureLoad.

Converting my u16 data to f32 on the CPU and then uploading that into a R32Float texture fixes the issue, no other changes required.

Repro steps

Using the WebGL backend:

1. Create a Depth16Unorm texture
2. Fill it with arbitrary data
3. Read from it in a shader

Expected vs observed behavior

Writing to and reading from a Depth16Unorm should work on both native and web, as the spec suggests.

Platform

• wgpu 0.15
• Linux (Arch), also reproduced on macOS
• Latest Firefox & Chromium

[teoxoy]

Looking at the spec (https://registry.khronos.org/webgl/specs/latest/2.0) it seems only those depth/stencil formats are supported:

• Depth32Float
• Depth32FloatStencil8
• Depth24Plus
• Depth24PlusStencil8

There are a few other open issues related to GLES texture format support.
The backend doesn't advertise support for DownlevelFlags::WEBGPU_TEXTURE_FORMAT_SUPPORT and as far as I know nobody researched what the gap is between the WebGPU spec, WebGL and GLES (since even between these 2 there seem to be differences).

[cwfitzgerald]

We should have built up texture flag support independently of the spec building off the gles specs. When WEBGPU_TEXTURE_FORMAT_SUPPORT is disabled, we should defer to the backends texture flags for our checks, but that obviously isn't happening here for some reason.