What Is WEBM? How the VP8, VP9 and AV1 Codecs Work

The web's own video format

While watching video on the internet you generally do not think about which file your browser is pulling in the background. But that file has a format, and it is most likely WEBM.

WEBM is an open video container designed for web browsers. Its reason for coming into existence was clear: for browsers to be able to play video, a format was needed that carried no licensing restrictions and that everyone could use freely. The most widespread video codec of the time, H.264, was tied to patent licences and created problems for open-source browsers. WEBM was born to fill that gap.

Today WEBM works almost everywhere inside the browser. Outside the browser the situation is very different — and to understand the reason for that difference, you first need to look inside the container.

WEBM is a container, and its contents vary

A video file consists of two layers: the container on the outside and the codec on the inside.

The container is the structure that holds the video track, the audio track, subtitles if there are any, and the timing information for all of them together. WEBM is a container. Technically it is a version of the Matroska (MKV) container restricted for the web — where MKV can carry anything, WEBM permits only certain open codecs.

The video codecs you can find inside WEBM:

• VP8: the first generation. Roughly the same era as H.264, at a similar efficiency.
• VP9: the second generation. Markedly better compression, broad browser support.
• AV1: the third generation. VP9's successor, more efficient but far heavier to encode.

On the audio side there is Vorbis (in older files) and Opus (in newer ones).

This list looks short, but it is deliberately so. WEBM's strength comes not from flexibility but from predictability: when a browser sees a WEBM it knows within a narrow range what is inside, and prepares accordingly.

How do codecs compress better?

The basic idea of video compression is this: consecutive frames are very similar to one another. If the camera is fixed in a scene and only one person is speaking, what changes between frames is a very small portion of the image. Rather than recording every frame from scratch, the codec stores the information of "what changed compared to the previous one".

The difference between codec generations lies in how well they can make that prediction. Newer generations can divide the image into blocks more flexibly, track motion in finer detail and use more complex prediction methods. This is why VP9 can deliver comparable quality with less data than H.264, and AV1 less than VP9.

But this has a price, and the price is very concrete: smarter compression means more computation. Encoding with VP9 is slow compared with H.264. Encoding with AV1 is markedly slower still. Even a short video can take a noticeable amount of time, and on long, high-resolution recordings the process runs to minutes. This is the fundamental trade-off you have to accept when working with WEBM.

WEBM's biggest problem: leaving the container

WEBM works perfectly in the browser. The problem starts when you leave the browser.

Your phone's gallery app generally does not recognise WEBM. Smart televisions mostly will not open it. Some video editing programs refuse to import it. Most social media platforms will not accept the upload. In messaging apps such as WhatsApp the file goes as a document, not as a video.

The reason for this is not a technical flaw but an ecosystem difference. Device manufacturers and platforms spent years building hardware and infrastructure around H.264; every phone chip has a dedicated circuit that decodes H.264. That support is newer and narrower for VP9 and AV1. So WEBM's constraint lies not in the format itself but in where the rest of the world put its investment.

Why is moving from WEBM to MP4 not lossless?

Here we come to a critical point. Some format conversions are just a container change: the data is taken out of one box and put into another, and the picture is never touched. This is called a remux and there is no quality loss.

That is not possible when moving from WEBM to MP4. The MP4 container does not carry the codecs WEBM holds in common usage — if you put VP9 video and Opus audio into an MP4, most devices cannot open the resulting file, meaning the operation you performed to gain compatibility fails to achieve its purpose.

For this reason the conversion always requires re-encoding: VP9/AV1 footage is decoded and recompressed as H.264, and Opus audio is converted to AAC. Moving from one lossy codec to another always loses some information. With good settings that loss stays at a level most viewers cannot detect, but it is technically there and it cannot be undone. The process also takes time.

There is one more side effect: the file usually gets bigger. Because VP9 is more efficient, holding the same quality with H.264 requires more data. The WEBM to MP4 converter you have to hand tries to keep this balance at a sensible point, but it cannot change physics.

When is WEBM a good choice?

Choose WEBM in this situation: the video is going to be published on your own website and viewers are going to watch it in a browser. In this scenario WEBM offers a smaller file, faster loading and an open format with no licensing headaches. It also supports an alpha channel, that is a transparent background — a feature not commonly available in MP4.

Stay away from WEBM, or convert: if the video is going to be uploaded to social media, watched from the gallery on a phone, played on a television, or taken into an editing program.

In summary

WEBM is an open container designed for the web's own needs, and it is genuinely good at that job. The VP9 and AV1 codecs inside it compress more efficiently than H.264, and in return exact a price in encoding time and device support. When you want to move to MP4, an unavoidable re-encode awaits you — a video that opens everywhere, in exchange for a small quality loss and probably a larger file. Which side you should stand on is determined by where the video is going.