[css-grid] Capping the size of grids?

The current spec allows implementors to cap the number of repetitions
allowed for the repeat() function[1]. I'd like to propose to extend this
to the values accepted by the grid positioning properties. The rationale
behind this is that those positioning properties might act as grid
definition properties (by creating implicit grid tracks), allowing
authors to do things like: "grid-row-start: ARBITRARY_HUGE_NUMBER;"

So my proposal would be to remove the special case of repeat() in the
specs and replace it by a more general statement allowing implementors
to cap the number of tracks, no matter if they're explicitly or
implicitly defined.

WDYT?

BR

[1] http://dev.w3.org/csswg/css-grid/#repeat-notation