EBML (Extensible Binary Meta Language) is a binary, tree-structured format — like a binary XML. Matroska (MKV) and WebM are both EBML documents. Every element has a variable-length ID, a variable-length size, and either child elements or a typed value.

How are MKV and WebM related?

WebM is a restricted subset of Matroska. Both use the same EBML container; WebM limits codecs to VP8/VP9/AV1 video and Vorbis/Opus audio and sets the EBML DocType to webm instead of matroska. The inspector reports the DocType so you can tell them apart.

How is duration stored?

Duration lives in the Segment Info element as a floating-point value measured in TimestampScale units (default 1,000,000 nanoseconds = 1 ms). The inspector multiplies duration by the scale and converts to seconds for you.

What are the common codec IDs?

Video codecs appear as strings like V_VP9, V_VP8, V_AV1 or V_MPEG4/ISO/AVC (H.264). Audio codecs appear as A_OPUS, A_VORBIS or A_AAC. Subtitle tracks use S_TEXT/UTF8 or S_TEXT/ASS. The inspector shows the raw CodecID for each track.

Is the video uploaded anywhere?

No. Parsing runs entirely in your browser with FileReader and DataView, reading only the header region of the file. Nothing is transmitted, so the tool works offline and your media stays private.

EBML / Matroska Inspector

Peek inside a Matroska (.mkv) or WebM (.webm) container without a media player — read its EBML header, doc type, segment info and every track’s codec, type and language. The inspector walks the EBML element tree directly with the browser’s DataView API, entirely on your device.

How it works

MKV and WebM are EBML documents — a binary tree where each element has three parts:

[ Element ID ][ Size ][ Data ]

Both the ID and the size are variable-length integers (vints). The first byte’s leading bits mark the length: a leading 1 means 1 byte, 01 means 2 bytes, 001 means 3 bytes, and so on. For the size, those length-marker bits are stripped before reading the value; for the ID, they are kept (the ID is used verbatim).

The inspector reads only the elements that carry metadata:

EBML header (0x1A45DFA3)
  DocType        (0x4282)  "matroska" or "webm"
  DocTypeVersion (0x4287)
Segment (0x18538067)
  Info (0x1549A966)
    TimestampScale (0x2AD7B1)  nanoseconds per tick (default 1,000,000)
    Duration       (0x4489)    float, in TimestampScale units
    Title          (0x7BA9)
    MuxingApp      (0x4D80)
    WritingApp     (0x5741)
  Tracks (0x1654AE6B)
    TrackEntry (0xAE)
      TrackType (0x83)   1=video 2=audio 17=subtitle
      CodecID   (0x86)   e.g. V_VP9, A_OPUS
      Language  (0x22B59C)
      Name      (0x536E)

Master elements (Segment, Info, Tracks, TrackEntry) contain children, so the inspector recurses into them; leaf elements hold a typed value (string, unsigned integer or float).

Duration

Playback length is computed from two fields:

seconds = Duration × TimestampScale ÷ 1,000,000,000

With the default scale of 1,000,000 ns, a stored Duration of 7200000 represents 7,200 ms = 7.2 s.

Tips

A DocType of webm guarantees web-playable codecs (VP8/VP9/AV1, Opus/Vorbis); matroska allows any codec.
The Segment element’s size is often unknown (all size bits set to 1) in streamed files — the inspector handles this by scanning to the end of the read region.
Track language defaults to eng when no Language element is present, per the Matroska spec.