I believe that this unfortunately blocks snapshotting the spec and test suite. It's an offshoot of the recently discussed issue about what getContextAttributes should return when the context is lost.
The WebGL spec has this IDL for getContextAttributes:
[WebGLHandlesContextLoss] WebGLContextAttributes? getContextAttributes();
And WebGLContextAttributes is defined to be a dictionary type:
But the WebIDL spec says that dictionaries cannot be nullable:
"The inner type MUST NOT be any, a dictionary type, [...]"
I believe that we should wait until this is resolved before we snapshot the spec or the conformance tests. One possible solution would be if WebIDL people agreed to make this restriction only apply to function parameters and not to return values.