Tests for moving focus by pressing tab key across shadow boundaries.
To manually test, press tab key sixteen times then shift+tab sixteen times.
It should traverse focusable elements in the increasing numerical order and then in the reverse order.

1. First sequentially focusable element
2. The focusable element in shadow tree with the higehst tabindex
3. The focusable element in shadow tree with the lowest tabindex
4. Slotted content with tabindex=4
5. Slotted content with tabindex=0
6. The focusable element in shadow tree with the higehst tabindex
7. Focusable slot 1
8. Content in slot 1 with tabindex=7
9. Content in slot 1 with tabindex=0
10. Content in slot 2 with tabindex=1
11. Content in slot 2 with tabindex=1
12. Content in slot 2 with tabindex=0
13. Non-focusable slot fallback with tabindex=1
14. Focusable slot element.
15. Shadow content with tabindex=2
16. Non-focusable slot fallback with tabindex=0
17. Focusable slot fallback content with tabindex=0
16. Non-focusable slot fallback with tabindex=0
15. Shadow content with tabindex=2
14. Focusable slot element.
13. Non-focusable slot fallback with tabindex=1
12. Content in slot 2 with tabindex=0
11. Content in slot 2 with tabindex=1
10. Content in slot 2 with tabindex=1
9. Content in slot 1 with tabindex=0
8. Content in slot 1 with tabindex=7
7. Focusable slot 1
6. The focusable element in shadow tree with the higehst tabindex
5. Slotted content with tabindex=0
4. Slotted content with tabindex=4
3. The focusable element in shadow tree with the lowest tabindex
2. The focusable element in shadow tree with the higehst tabindex
1. First sequentially focusable element