Can you share the iPhone/iOS/Safari version? I've not heard of that particular issue before.
I know some browsers show a default border for images and/or links under some conditions. The Flarum reset CSS is supposed to clear those browser-specific styles before the Flarum layout is applied. But older browsers might no longer be covered by our reset stylesheet. Flarum is currently using Normalize.css v8.0.1 for its reset CSS https://github.com/flarum/framework/blob/main/framework/core/less/common/normalize.less I'm not seeing any rule for image tags in Normalize.css though.
Do you see the border around the logo when you visit this website? (discuss.flarum.org) If you don't see the border here, it might be caused by an extension or custom CSS you have applied.
