Source order and visual order

Click into the page, then press Tab to move through the links. Each link shows its position in the source.

1. Source 1
2. Source 2
3. Source 3
4. Source 4
5. Source 5
6. Source 6

With the box checked, the eye reads the grid row by row, but focus still visits Source 1, 2, 3 and so on, jumping around the screen.