Which grid-template-areas value is invalid, causing the declaration to be ignored?

"a b" "b b" is right. Each named area must form one filled rectangle. Here area b is L-shaped: one cell in the first row and two in the second. So the whole value is invalid, and the browser ignores it.

"head head" ". main" may look like the wrong one, but it is valid. A . marks an empty cell, which is allowed.