Why is my transparent WebM black in Safari?

Safari on macOS: docs say it can play VP9 WebM but does not composite the alpha (WebKit bug #275908: grey background rather than transparent). Chrome, Firefox, and Edge keep alpha (tested). Your WebM file is likely correct. We have not tested Safari ourselves; iOS WebM alpha is not verified.

Export WebM for Chrome, then handle Safari separately

This tool writes VP9 WebM with alpha for Chrome, Firefox, and OBS. Safari still needs HEVC-with-alpha.

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Safari plays WebM but doesn't composite alpha

Important: Safari does support WebM playback. What it doesn't support is compositing the alpha channel. This is a critical distinction. Don't write "Safari doesn't support WebM"—that's incorrect. Safari plays VP9 WebM video; it simply doesn't composite the alpha layer. Prefer grey background (WebKit quote) over black when describing Safari×WebM alpha.

WebKit bug #275908 tracks this; status was NEW when cited for our 2026-09-24 matrix. See the full alpha compatibility matrix for what works where.

The Safari solution: HEVC-with-alpha

Per Apple WWDC19 session 506: HEVC video with Alpha is supported by Safari in iOS 13 and macOS Catalina. This format stores transparency in a separate HEVC layer. Safari (and other Apple platforms) can composite it. We did not play-test HEVC-with-alpha (no true encode on Linux). Verify on your target OS version.

Serving both formats for cross-browser support

Use the HTML5 video element with multiple source tags:

<video>
  <source src="video.mov" type='video/mp4; codecs="hvc1"' />
  <source src="video.webm" type="video/webm" />
</video>

List HEVC first. Safari will pick it. Chrome and Firefox will skip it (no HEVC support or licensing) and fall back to WebM. This ensures transparency works in both browsers.

Capability detection

Don't rely on user-agent sniffing. Use the Media Capabilities API to test whether the browser can decode and display a given codec. Older Safari versions may report HEVC support but not composite alpha. Capability detection is more reliable than UA strings.

Frequently Asked Questions

Why doesn't Safari just support WebM alpha like Chrome?

Apple prioritizes its own codecs (HEVC) over WebM. WebKit can play VP9 video but doesn't implement alpha compositing for it. The decision is Apple's; the WebKit bug remains open.

Will Chrome on iOS support WebM alpha?

Chrome, Firefox, and other third-party browsers on iOS use WebKit under the hood (Apple requirement), so they share Safari's engine. iOS WebM alpha is not verified in our matrix — we do not assert whether those browsers composite WebM alpha on iOS. For Safari transparency, use HEVC-with-alpha per Apple docs.

Can I polyfill WebM alpha in Safari with JavaScript?

Theoretically yes, by decoding video frames to canvas and manually compositing alpha. This is complex, CPU-intensive, and not practical for most use cases. Use HEVC-with-alpha instead.

Sources

Export for Chrome, Firefox, and OBS

Upload your video and get transparent WebM with alpha. Use HEVC-with-alpha if the page must work in Safari.

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