Transparent WebM Black or Grey in Safari

What the sources say

Four primary sources cover this. We opened each one on November 9, 2026, and the table lists only what the pages literally say.

Two details matter for the fix. The WebKit report is about VP9 with transparency, it was still open (status NEW) when we checked, and it does not name a fixed version. Apple’s talks cover HEVC with alpha, which is a different codec path from VP9.

What we measured in Safari, Chrome and Firefox

We cut a 5 second, 1280×720 clip out of our own demo footage and saved it twice: as a VP9 WebM with alpha (1.0 MB) and as an HEVC-with-alpha .mov (2.3 MB). On November 9, 2026, on one Mac running macOS 15.6.1, we loaded both files in each browser and read back the top-left pixel of the first frame. That pixel sits in the transparent area, so alpha 0 means the browser kept the transparency and alpha 255 means it did not.

In Safari 18.6 the WebM came back opaque, with the color (118, 115, 104) from underneath the transparent area. That is Safari ignoring the alpha channel and showing whatever the file stores there, which is why people report grey or black depending on the file. Only the file’s own pixel values decide which.

Check that the file itself is fine

Before you change anything, confirm the WebM really has an alpha channel. If it is transparent in Chrome or Firefox over a colored page, as in the table above, a Safari failure is not a bad export.

You can also read the file’s metadata. A VP9 WebM with alpha keeps the alpha plane separate, so the first command reports a normal pixel format plus an alpha_mode tag. The second command decodes the file with libvpx and shows the real format.

ffprobe -v error -select_streams v:0 -show_entries stream=pix_fmt:stream_tags=alpha_mode -of default=nw=1 clip.webm
ffmpeg -hide_banner -c:v libvpx-vp9 -i clip.webm -frames:v 1 -f null -

Black or grey: which problem do you have?

People say “black”, but the causes are different and need different fixes. Use the symptom to pick the row.

Fix 1: give Safari an HEVC file with alpha

Apple’s sessions say HEVC with alpha works in Safari on iOS 13 and macOS Catalina and later, and our test above confirms it on Safari 18.6. Put the HEVC file first with a codecs type, and the WebM second. Browsers that cannot play the HEVC type skip it: in our test Firefox and Chrome moved on to the WebM, and Safari used the HEVC file.

You need a source that already has transparency, for example a transparent WebM or a ProRes 4444 file. On a Mac, FFmpeg can encode HEVC with alpha through Apple’s VideoToolbox. The -alpha_quality option appears in ffmpeg -h encoder=hevc_videotoolbox (range 0 to 1, default 0). The command below made the 2.3 MB HEVC file we tested.

ffmpeg -c:v libvpx-vp9 -i clip.webm -c:v hevc_videotoolbox -alpha_quality 0.75 -tag:v hvc1 clip-hevc.mov

The HTML that served each browser the right file

This is the exact pattern we tested. Both files sit in one video element, HEVC first. Keep autoplay, muted, loop and playsinline so it plays on iPhone.

It worked in all three browsers on one Mac. We did not test iOS, Windows, Android, Edge, or desktop Chrome with hardware HEVC decoding, which may accept the HEVC type. If a browser picks the HEVC file and shows no transparency, switch to choosing the file with a script that checks for Safari.

<video autoplay muted loop playsinline>
  <source src="clip-hevc.mov" type='video/mp4; codecs="hvc1"'>
  <source src="clip.webm" type="video/webm">
</video>

Fix 2: skip transparency if the page background is a flat color

If the video sits on a plain page background, you do not need an alpha channel at all. Export an MP4 on a solid color that matches the page and it plays the same everywhere, Safari included. The upload tool can export MP4 on a preset color, any color you pick, or your own image. For a transparent sticker or chat reaction, a transparent GIF avoids the VP9 problem, but it only has fully transparent or fully opaque pixels.

What we exported, and what we did not test

RenderAlpha exports transparent WebM, ProRes 4444 (clips up to 10 seconds), transparent GIF and MP4 on a color or image. It does not export HEVC with alpha, so use the steps above to make that file. See specs and limits for the numbers.

Our browser results come from one Mac and one pair of test files. Treat them as a starting point and test your own file on the devices your visitors use.

Frequently asked questions

Sources

Primary sources we checked. The date is when we last opened each page.

• MDN: Web video codec guide: Safari and alpha in VP8 and VP9 (Retrieved November 9, 2026)
• WebKit bug 275908: VP9 with transparency plays back incorrectly: grey background, status NEW (Retrieved November 9, 2026)
• Apple WWDC19 session 506: HEVC Video with Alpha: iOS 13, macOS Catalina (Retrieved November 9, 2026)
• Apple WWDC19 session 518: What’s New for Web Developers: alpha video in the video element (Retrieved November 9, 2026)