You definitely need a gif or apng file showing it's use in the github readme.

And a link to an asciicinema would help a lot too.

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Also, I'm not sure how useful the side-by-side view is.

The second example (https://github.com/flamestro/deff/blob/main/docs/example_02....) is confusing.

The left side has lines 1365-1371 having the same code as lines 1374-1380 on the right side, yet they're not aligned with each other.

Most diff views would put padding between lines 1364-1365 on the left side so lines 1365-1371 are aligned with 1374-1380 on the right side.