Keep the edge sample small
A transparent animation can look clean over one background and show a halo over another. Prepare a short owned sample containing soft edges, shadows and motion blur. Keep the original and record the export and receiving application's alpha expectations.
FFmpeg documents straight and premultiplied alpha choices in compositing. Those interpretations matter to how colour and transparency combine. A mismatch is a question to investigate, not a diagnosis for every dark outline.
Do not repaint the edge immediately to hide it against one colour. That may bake a compensating defect into the artwork and make a different background worse. First establish whether the source and receiving path agree about the representation.
Compare light, dark and real footage
Composite the same candidate over a light background and a dark background, then representative programme footage. Keep scale and colour handling fixed. Observe the edge during movement as well as in a paused enlargement.
If one documented interpretation change resolves the candidate consistently, record the requirement in the handoff. If it does not, inspect the artwork and export path separately. An alpha-bearing pixel format alone does not establish that every stage preserved the intended edge colours.
Deliver the actual tested candidate and its destination context. Keep the clean source and avoid claiming universal codec or player support from a compositing preview. The result should explain how the animation is intended to be imported, with any remaining edge limitation visible.
Questions answered
Does a halo prove the source artwork is poorly drawn?
No. First check representation and compositing assumptions.
Is a test over black alone enough?
Use contrasting backgrounds and representative underlying footage to expose different edge behaviour.
Sources & further reading
Official references for the subjects discussed. Service terms and device software can change.