CSS-only photo grid with horizontal fill

Question

I've found a workable CSS-only photo-grid:

https://css-tricks.com/seamless-responsive-photo-grid/

But the fill is vertical; that is, photo #2 is underneath photo #1 in the same column, rather than to the right of it. What I want is horizontal flow.

I can do this easily with JavaScript, but is there a way to do it with pure CSS?

Requirements:

1. Wrapping must be automatic; I don't want to assign photos to rows.
2. Left and right justification. The height of a row varies so that the photos in that row are flush to the left and right margins of the container.

3. The HTML should be extremely simple, something like this:

   <div class='photo-container'>
   <img src='...'>
   <img src='...'>
   <img src='...'>
   ...
   </div>

4. Aspect ratio of photos must be maintained. [UPDATE]

Any ideas?