The header accessibility trait makes VoiceOver announce something as a header. More importantly, it lets the user navigate through headings, swiping up/down, significantly speeding up navigation and making it easier to explore/discover content.
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The .selected accessibility trait indicates when an element has been selected. You’ll notice that VoiceOver announces “selected” before the accessibility label. You can find that in the system for the selected tab in the tab bar, for example.
Touch target sizes are recommended to be at least 44 x 44 points. Buttons in the navigation bar ( especially when not using nav bar button items), dismiss buttons, and custom toolbars, are use cases that tend to have smaller sizes.
A common example where you need to manually configure the button accessibility trait is for some table/collection view cells. These tend to be “buttons” that perform an action, like playing music, or bring the user to a different screen.
