The problem is mostly the Apple devices which don't support non-proprietary formats. H.264 is probably the most universal codec, but it's proprietary and requires a license for commercial use. The non-proprietary VP8 codec is supported by most browsers, but Safari support (outside of WebRTC) requires version 14.1, which was only released this year. And that's only on Mac OS, not iOS—though we might see WebM support for mobile Apple devices in the next release. And then there are complicating factors like "Edge support for VP8 requires the use of Media Source Extensions" and "Firefox only supports VP8 in MSE when no H.264 hardware decoder is available"[0] which stand in the way of a one-size-fits-all solution.

[0] https://developer.mozilla.org/en-US/docs/Web/Media/Formats/V...

Is there a format that is not proprietary and works for this purpose?

AV1 is the proposed future. But Apple has been holding back universal free formats for a while now. Apple is on the AV1 group so in theory they should support it later but they do not yet.