It's a bit of a mess. The implementation of a codec, that is, and encoder or a decoder can be open source, despite the format itself not being open. H265 does have open implementation, but the format itself is not open. The opposite can be true as well, there are proprietary encoders to open formats for example. Actual list of open video formats:
https://en.wikipedia.org/wiki/List_of_open_file_formats#Vide...What OP meant is that they would like an open format on the website, which, then, can be viewed in any modern browser. I think that caniuse is a good resource in this regard.
https://caniuse.com/av1
https://caniuse.com/hevc
WebM with VP9 video is a good general browser target I think:
https://caniuse.com/webm
But funnily enough, even though h264 is not open, it's a widely decoded video format as well:
https://caniuse.com/mpeg4