This is a confirmed regression from http://trac.webkit.org/changeset/136967.
The floats are aligned in Firefox and Opera and before the change. Not sure if it has any relation with the bug but it seems that there is some overlap of the background box.
Created attachment 187662 [details]
Comment on attachment 187662 [details]
View in context: https://bugs.webkit.org/attachment.cgi?id=187662&action=review
> // The top margin edge of a self-collapsing block that clears a float intrudes up into it by the height of the margin,
> // so in order to place this child float at the top content edge of the self-collapsing block add the margin back in before placement.
It may be worthwhile updating the comment since it's here. "in order to place the first child"
Created attachment 187911 [details]
Comment on attachment 187911 [details]
Clearing flags on attachment: 187911
Committed r142659: <http://trac.webkit.org/changeset/142659>
All reviewed patches have been landed. Closing bug.
For the next time, it would have been nice to update the bug title to something precise. My description was inaccurate as I didn't know what was wrong.
*** Bug 110274 has been marked as a duplicate of this bug. ***