Transparency must survive every stage
A transparent source image does not prove that the exported video will contain transparency. The preparation frames, chosen pixel representation, encoder, container and recipient application all take part in the handoff. Record the exact destination requirement before choosing a format from a generic list.
Treat alpha as a required property in the delivery contract. Include a sample with fully transparent space, opaque content and partially transparent edges. A logo containing only a hard cutout can conceal problems that soft shadows or motion blur reveal.
Separate raw representation from codec support
MDN's VideoFrame format documentation distinguishes alpha-carrying frame representations from opaque representations that contain padding. This is a useful conceptual boundary, but a raw-frame format is not a promise that every encoder or player preserves that property. Verify the actual processing path.
For an owned animation, prepare a short candidate and inspect the resulting media with a tool appropriate to that format. Then import it into the real compositing destination. A video player showing black behind the artwork cannot by itself establish whether alpha was discarded or merely displayed against black.
Review edges over useful backgrounds
Composite the candidate over both a light and a dark background, plus representative underlying footage. Check soft edges, shadows and any semi-transparent detail during motion. A visible halo can relate to the compositing interpretation as well as the artwork itself; do not immediately repaint the source to hide it on one background.
Keep the alpha interpretation documented where the workflow distinguishes straight and premultiplied data. The receiving application must use the matching expectation. A correction that looks right on one flat colour may still fail when the underlying picture changes.
Offer an explicit fallback
If the required destination cannot carry transparency, agree on a composited version for a known background or another supported production asset. Label it clearly. A flattened version is useful when its purpose is known, but it should not replace the only alpha-bearing master.
VideoFrame: format property by Mozilla Contributors is available under CC-BY-SA 2.5 or later; no code or documentation passage is reproduced here. The original review workflow does not establish universal browser or device support. Preserve the test sample, output identity and actual destination result for future handoffs.
Questions answered
Does a black player background prove alpha is missing?
No. Check the media property and test compositing in the intended destination.
Does a transparent PNG guarantee transparent video output?
No. The complete export and import path must preserve alpha.
Sources & further reading
Official references for the subjects discussed. Service terms and device software can change.