When using the tab key to navigate websites using the keyboard, certain elements do not get focused. The bug can be easily reproduced on this test website: https://wicg.github.io/focus-visible/demo/ In my testing I cannot get anything before the LINKS heading to be focused. I'm also unable to focus sliders or the file input.
All of these get selected if you use Option-Tab, or if you change a Safari advanced preference for Tab behavior, or if you enable full keyboard access in system preferences.