[gmueckl]

Ribbons (in Office, at least) change that very hierarchy depending on window width. For example, take the Home bar in Word: the "Editing" and "Style" groups collapse to buttons with submenus before the width of the window is reduced to half screen width on my computer. This affects keyboard navigation in the ribbon bar, too, of course.

[ModernMech]

This doesn’t actually change the hierarchy of commands, as evidenced by the fact the keyboard shortcuts stay constant. I appreciate hiding these first before Font, since I use Font more than Styles or Editing.

[gmueckl]

What hierarchy is supposedly staying constant? Keyboard shortcuts have no hierarchy.

[ModernMech]

Keyboard shortcuts for functions in the ribbon map directly to their placement in the ribbon hierarchy. Tab, section, subsection, etc. You press alt and then a series of one or more letters that define the path to that item.