The accessibility guide · Getting around the page
A clickable button jammed in the strip above the nav
A button in the strip above the nav either forces keyboard users through your promo first, or yanks their focus somewhere they didn't expect.
The choice you’re proud of
A clickable button jammed in the strip above the nav
What it actually is
A promo or “Book” button tucked into the thin bar at the very top.
Who it shuts out
Keyboard users move through a page top to bottom, in the order it's actually built. A clickable thing above the nav either forces everyone through your promo before they're allowed to navigate, or the focus box suddenly leaps to the top of the page and drops them somewhere they didn't expect — and they lose their place.