The theme review team is requiring all themes to implement keyboard navigation in two months time, around the third of september 2019.
Because this may be a complex task for many authors, we encourage you to start planning for and working on this as soon as possible.
We want to point out that this requirement does not only include menus. All functionality should work using a keyboard only. Any action you can complete with a mouse must also be performable via keyboard.
Theme authors must provide visual keyboard focus highlighting in navigation menus and for form fields, submit buttons and text links. All controls and links must be reachable using the keyboard.
First, please read the requirement found here: https://make.wordpress.org/themes/handbook/review/accessibility/required/#keyboard-navigation
As well as the background for this requirement: Techniques for WCAG 2.0
G202: Ensuring keyboard control for all functionality
We would like to collect examples for the keyboard navigation, including menus. If you know of a good resource that is not already included, please add a comment and link.
Navigation menus and controls
Drop down menus and controls must be available when tabbing, using keyboard only.
Both forward and backwards tabbing must work. To test backwards tabbing, hold down Tab and Shift at the same time.
Controls also include open and close buttons for modals such as off canvas sidebars or search forms.
Don’t break the browsers default focus by removing the outline. Other kinds of decorative changes are also allowed. Only showing the marker inside the field is not enough.
Submit buttons may be hidden during it’s normal state but visible on focus.
A high contrast color change, an outline, a border, or a text decoration change will assure that visible submit buttons pass the requirements.
Make sure you can see visually which link is focused. Focus should either incorporate a visual change that is not based on color (background change, underline, shape) or a color change where the difference between the two colors meets color contrast guidelines.
Basically, don’t break or remove the browsers default focus by removing the outline.
For those interested in AMP: