This came up earlier in discussions with @jensimmons and again when she was creating demos: spanning items create gaps above them, and it would be useful to be able to align within those gaps. In particular, applying stretch or end alignment would improve the effects in demos such as https://webkit.org/demos/grid3/photos/ (e.g. in "Every 5th item").

The proposal is to use the self-alignment properties in the masonry axis to align an item above a spanner or at the end of the masonry grid in the space allotted to it + the subsequent gap.