Webflow’s collaborator login requires third-party cookies to function. Safari blocks third-party cookies by default because they are a privacy nightmare.
The recommended workaround is for collaborators to disable the “Prevent cross-site tracking” feature in Safari’s Privacy preferences. But I would frankly sooner recommend people look for alternatives to Webflow than to compromise their online privacy in this way.
Technically the issue is that published Webflow sites exist under the webflow.io domain but the API used by collaborator logins is under webflow.com. Because the domains do not match, cookies set by webflow.com are third-party and thus blocked in Safari.
People have been complaining about this on the forum since 2018. It’s surprising that Webflow still doesn’t seem to consider this an issue. Macs, iPads and iPhones are pretty common these days.