custom_nodes/ComfyUI-tbox/nodes/video/video_formats/webm.json

{
    "main_pass":
    [
        "-y",
        "-crf", 20, 
        "-pix_fmt", "yuv420p",
        "-b:v", "0"
    ],
    "audio_pass": ["-c:a", "libvorbis"],
    "extension": "webm"
}