[css3-grid-layout] mismatched grid template strings

Lea Verou pointed out a problem in the Grid Layout spec: it doesn't
specify what happens if the template strings are not all the same
length, e.g.

   grid-template: "aa"
                  "b"
                  "bg";

Possible options are to treat this as invalid; or to fill the missing
slots with "." (the Template module's unnamed placeholder).

~fantasai