[ahoy]

You're halfway there. The indices are the grid lines, but they're also one-indexed. That's consistent with other parts of CSS, though it's inconsistent with the rest of the programming world.

[goatlover]

Julia, R, Matlab, Basic and Fortran disagree.

It's just that C's heavy influence resulted in most languages with arrays being 0 indexed.

[fineline]

APL disagrees too. Except when it doesn't.