Why ProRes 4444 rather than WebM
WebM is a delivery format for browsers. Many editors do not import it natively, or ignore its alpha when they do. ProRes 4444 is an editing codec with a full alpha plane that Premiere Pro, After Effects, DaVinci Resolve and Final Cut Pro all read. Every video result here can be exported as ProRes 4444 for free, and the WebM to ProRes converter handles files you already have.
Straight or premultiplied?
Alpha comes in two flavours. Straight (unmatted) keeps full colour in semi-transparent pixels; premultiplied has the colour already mixed with a background. If the editor guesses wrong you see dark or light halos around hair and soft edges. FFmpeg treats alpha as straight, so start with Straight and switch to Premultiplied only if the edges look wrong over a solid colour.
After Effects and Premiere Pro
In After Effects, select the footage in the Project panel → File → Interpret Footage → Main → set Alpha to Straight – Unmatted. Premiere Pro reads the alpha of a ProRes 4444 file automatically; place it on a track above your background. If you want to be sure, put a solid colour underneath: transparent areas show the colour, not black.
DaVinci Resolve and Final Cut Pro
In DaVinci Resolve, right-click the clip in the Media Pool → Clip Attributes → set Alpha Mode to Straight. Final Cut Pro imports ProRes 4444 with its alpha; put the clip above your background in the timeline.
Why it looks green or black in a normal player
QuickTime Player and most media players do not show transparency. A transparent clip can look black—or show leftover colour—there even though it is fine. Judge the file in your editor over a coloured background, not in a player.