Section 11.1 of the WGSL spec says:

WGSL defines the following attributes that can be applied to function parameters and return types:

• builtin
• location
• blend_src
• interpolate
• invariant

I'm not sure why blend_src is on this list? In an indirect way, it does apply to fragment stage outputs, if it's applied to a struct and that struct is an output, but it seems like @align and @size could be said to apply in the same way?