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And two thirds, and three fifths, …

good idea

Multiply your rating system by 2 and you won't need half stars :)

This makes subitizing much more difficult though.

This comment taught me a word. Separately, you are completely correct and this is extremely valid in the design of rating systems.

Only the star with the left side filled in. And an outline on the right.

This is the role of digits I believe.

What if my products are rated in smiley-faces?

I think something like an occlusion mask modifier (slice off this much from this side/corner) would be more useful.