Tangent vector equivalent for Canvas shader graph

[shy palm]

I'm trying to implement the algorithm from this thread for creating an outline shader for procedurally-generated polygons in a Canvas:
https://discussions.unity.com/t/shader-to-create-an-outline-inside-a-polygon/817698/3

However, it looks like a Canvas mesh can't have tangents:

Is there a way around this? In my case I might just be able to use UV coordinates since I'm not using them for anything else, but I am wondering why Canvas vertices don't have a tangent in the first place.