WebM is a video and audio container format released by Google, used with content encoded under the VP8 or VP9 codecs for video, and Vorbis and Opus for audio. It is a variant of Matroska, based on EBML.
A WebM file begins with
0x1a 0x45 0xdf 0xa3, as do all EBML files.
It contains an EMBL "DocType" element whose value is the string "webm". This means that the byte sequence
0x42 0x82 0x84 'w' 'e' 'b' 'm' will appear in the file, normally near the beginning.