For bonus points you've probably broken accessibility on many platforms as the way that a11y works is intrinsic to the native widgets used by a platform.
I wouldn't assume that. It's literally the number one thing people bring up with custom UI components, I'd be very surprised if Flutter ignored the issue.
Oh I'm sure they didn't ignore is, just that you're constrained to platforms that allow a side-channel mechanism to expose a11y and handle focus+navigation.
I haven't looked deeply at the HTML5 APIs but if you wanted to say port Flutter to HTML5 Canvas you may run into problems where you can't expose rich enough a11y primitives on something that was just meant to be a bitmap.
