WebM - Open Web Media File

WebM is a video format Google introduced in 2010 with one goal: video that a web browser could play without anybody paying a patent licence. The codecs used in MP4 at the time were covered by patent pools, which made them awkward for open-source browsers and for anyone publishing video at scale. Google took the open Matroska container, cut it down to a small, strictly defined subset, paired it with royalty-free codecs and published the result. Today WebM is everywhere on the web without most people noticing, and you meet the files directly in two ways: by downloading a video from a site, and by using a screen or webcam recorder that runs inside a browser, because that is the format browsers record in.

How a WebM file is put together

Structurally, WebM is Matroska with the guest list shortened. It uses the same EBML layout, a binary nesting structure similar in spirit to XML, and the same idea of a segment holding tracks and clusters of timestamped blocks. The difference is what is permitted inside. Video must be VP8, VP9 or AV1; audio must be Vorbis or Opus. All of those are lossy, and the newer ones squeeze more quality into the same number of bytes at the cost of a slower encode. VP8 and VP9 can also carry an alpha channel, so a WebM can have genuine transparency, which is handy for overlays and animations on a web page.

Because the format is defined so narrowly, a WebM file that plays anywhere plays everywhere that supports WebM at all, and browsers can decode it without extra software. The same narrowness means a WebM cannot hold the many optional extras a full Matroska file can.

What metadata a WebM file carries

Very little, and that is deliberate. A WebM is usually one of the quietest video files you can be handed.

What is reliably there is technical. The segment information gives the duration and often a creation date in UTC, plus a muxing application and a writing application: the names of the library and program that produced the file. A recording made in a browser typically shows the browser as the writer, and a converted file usually names the conversion library. Each track entry states the codec, the pixel dimensions, the frame rate where it is stored, a language code and the audio channel count and sample rate.

Descriptive tags are possible. WebM inherits Matroska's tag system, so a file may hold a title, an artist, a description or an encoder note, grouped by what they apply to. In practice they are often absent, because most WebM files are produced automatically by a website or a browser rather than tagged by a person.

What WebM does not have is any camera metadata at all: no EXIF block, no camera make or model, no GPS coordinates, no lens or exposure fields. Nothing in the format is designed to record where a video was shot. There is one thing to watch, though. If a WebM was made by converting a phone recording, the conversion tool may have copied titles, dates or comments across from the original, so it is worth reading the report rather than assuming the file is empty.

Browser recordings have a quirk of their own: the file is written while the recording is still running, so the duration is sometimes missing or zero. That is why some players show a screen recording with no timeline until the file has been remuxed.

When to choose WebM

Choose WebM for video you are putting on a web page, especially if you want transparency or you would rather not think about codec licensing. File sizes at a given quality are good, and no plugin is needed.

MP4 with H.264 remains the safer choice for anything leaving the web: older televisions, set-top boxes, video editors and some phone galleries handle it more reliably, and hardware decoding for it is close to universal. If you need one file that works for everyone, MP4 is the pick; if you control the page it plays on, WebM is the lighter, freer option.