How WebM Works
WebM is an open video format built by Google specifically for the web, and it is best understood as a purpose-built subset of Matroska (MKV). It reuses Matroska's EBML-based container structure but deliberately restricts the codecs it allows to royalty-free choices only: video must be VP8, VP9, or AV1, and audio must be Vorbis or Opus. That restriction is the whole point. By permitting only open codecs, WebM stays completely free of the patent-licensing requirements attached to H.264 and AAC, the codecs typically found in MP4.
Like every modern video format, WebM is a container, not a codec, but its codec list is constrained on purpose. It was designed to be the native, license-free video format for HTML5 streaming, so that browsers could play video with the <video> tag without anyone paying codec royalties. Because it inherits Matroska's design, WebM also supports useful features like multiple tracks and good streaming behavior, while staying lean enough to be ideal for delivering video over the web.
In practice, WebM's efficiency depends on its codec. VP9 is roughly comparable to H.265 in compression, and AV1 (which WebM also supports) is more efficient still, so a WebM can be noticeably smaller than an H.264 MP4 of the same quality. This combination of small size and zero licensing cost is exactly why platforms like YouTube serve large amounts of video as WebM behind the scenes.
WebM vs MP4 on the Web
On the web, WebM and MP4 are the two formats the HTML5 <video> element commonly uses, and they are complementary rather than competing. WebM offers better compression and no licensing fees, which appeals to large platforms watching their bandwidth and royalty costs. MP4 (H.264) offers the broadest hardware support, playing with hardware acceleration on virtually every device, which is gentler on battery life and works on older hardware. Many sites therefore provide both, letting the browser pick whichever it supports, often preferring WebM where available and falling back to MP4.
For a creator, the practical takeaway is that WebM is excellent for video embedded on your own website, where you control the markup and can offer a fallback, but it is not the format to hand someone for general use. Outside the browser, WebM support is patchy: many video editors, messaging apps, and devices do not handle it, which is why a WebM you want to share or edit is usually best converted to MP4 first.
When to Use WebM
Use WebM for web video delivery: background videos, embedded clips, and HTML5 players on your own pages, where its small size and royalty-free codecs are real advantages and you can supply an MP4 fallback for browsers that need it. It is also a sensible choice when you specifically want an open, license-free format and your audience uses modern browsers. For sending video to other people, uploading to platforms that expect MP4, or editing in most software, convert to MP4 instead.
Limitations
WebM's limitations are all about reach outside the browser. Support is uneven in non-browser software: many video editors, media players, phones, and messaging apps do not open WebM cleanly, so it is poor for general sharing. Its hardware acceleration is also less universal than H.264's, particularly on older devices, which can mean higher CPU use during playback. None of this affects its core mission, efficient, royalty-free web video, but it does mean WebM is a delivery format for the web rather than an everyday file you pass around.
