What Is WebM? The Open-Source Video Format Explained

What is WebM?

WebM is an open video format developed by Google and released in 2010, the same year as WebP. It was designed to provide an open, royalty-free alternative to MP4/H.264 for the web. WebM uses the Matroska container format with VP8, VP9, or AV1 video codecs and Vorbis or Opus audio codecs — all open standards with no licensing fees.

Google developed WebM to reduce the web's dependence on MPEG-licensed codecs (H.264, AAC), which require license fees from manufacturers and developers. By providing a fully open alternative, WebM ensures that any browser or device can support web video without paying royalties.

How WebM works

WebM is a container — it wraps encoded video and audio streams:

• VP8: The original WebM codec (2010). Comparable to H.264 baseline. Now rarely used for new content.
• VP9: Published in 2013. Significantly more efficient than H.264 — comparable to H.265 HEVC without the licensing cost. The current standard WebM codec.
• AV1: Published in 2018. The next-generation open codec — 30–50% smaller than H.264 at equivalent quality. Slow to encode, but increasingly supported for streaming.
• Audio: Vorbis (legacy) or Opus (current). Opus is technically superior to AAC at low bitrates.

Like MP4, WebM uses temporal compression between frames. VP9 and AV1 achieve better compression than H.264 by using larger prediction blocks, more sophisticated motion estimation, and more flexible entropy coding.

File size

VP9 WebM is typically equivalent to H.265 HEVC MP4 and noticeably smaller than H.264 MP4 at the same quality. AV1 WebM pushes further:

Browser and OS support

Outside browsers, WebM support is patchy. VLC and most open-source media players support it. Windows Media Player does not natively. For maximum compatibility, always pair WebM with an MP4 fallback:

<video autoplay loop muted playsinline>
  <source src="video.webm" type="video/webm">
  <source src="video.mp4" type="video/mp4">
</video>

When to use WebM

• Web-first video delivery: When you control the playback environment (your own website), WebM with an MP4 fallback gets the best compression to all modern browsers.
• YouTube and streaming: YouTube serves VP9 WebM to Chrome/Firefox browsers and H.264 MP4 as a fallback — the same dual-format approach.
• Cost-effective storage: Smaller files mean lower CDN and storage costs when serving large amounts of video.

If you only want to maintain one video format, H.264 MP4 is still the safer choice. WebM's advantage is file size; its disadvantage is slightly less universal support, especially on older iOS.