Visual Studio doesn't seem to like SPAN tags. I've already reported a bug where it adds duplicates SPAN tags to templated fields in a GridView or DetailsView that is within a span tags. Now, it's moving my span tags around.
I had two span tags to define left and right columns with inline-block. They were contained within the same div. Somehow, VS decided that the left span belonged to the div above and moved it all on it's own. If it keeps happening, I'll see if I can narrow the circumstances down and get a bug report filed.