When the @interpolate(flat) attribute is applied to an interstage variable in WGSL, the rasterizer does not interpolate the value of that variable, and assigns a single value to all its pixels. The value assigned to all pixels is determined by the attributes of a single vertex, called the provoking vertex.

In OpenGL, the last vertex in each primitive is the provoking vertex (see OpenGL ES
Version 3.1 section 12.3, "Flatshading").

In all other APIs, including WebGPU, the first vertex in each primitive is the provoking vertex. Desktop OpenGL 4.0 has a glProvokingVertex to select the default, but OpenGL ES does not.