Thanks for bringing this backup. I did some research on it yesterday, and it appears the Chrome browser supports it but not Safari. That is the case with Slack too if you are running it through the browser, not the app.
I haven’t checked Firefox or IE yet but because it’s not universally supported it’ll have to have some sort of fallback. Right now I think this will need to be a v5 feature because it’ll need quite a few substantial internal changes and a smooth system in place for when a browser isn’t supported.