WEBM stays WEBM, with VP9 video
A WEBM file is compressed to WEBM by default. The video is encoded with VP9 and the sound with Opus, the codecs WEBM is built around. The plan shows this as "WEBM · VP9".
WEBM output is written by your browser's own video encoder. If your browser cannot encode WEBM, the job cannot finish in that format. In that case, ask for an MP4: ToolGPT can also write MP4 with its built-in FFmpeg when the browser cannot.