See testcase: where no explicit width is specified for the grid container, its intrinsic width (used during inline layout) returns zero, as shown by its border, by the overlapping content in the first example, and by the lack of a line-break before it in the second. When the grid container's width is specified, we get the layout I'd expect (3rd and 4th examples). (I wasn't sure if this may be covered as part of bug 1151212 or others; if so, feel free to dupe it accordingly.)

Nightly doesn't have intrinsic or flex sizing yet. It only has step 1 of http://dev.w3.org/csswg/css-grid/#algo-track-sizing Step 2, 3 and 4 is in my patch queue and is covered by bug 1151212. (your example renders as expected locally) I'll start submitting patches for review next week (it's the remaining part of my goals for Q2).

Sounds good, thanks!