After the 8.3 update, I am getting an extra <> on my page as shown in https://db.tt/w9MpZu8RAR
The <> appears just below a views slideshow block, but remains displayed even if the block is disabled. It also is independent of theme.
The theme I use (ZURB Foundation) uses no main-content block on its front page. This is the origin of the spurious "<>" on my page. When I display the main-content block (which was not displayed previously) in the region Left, off page, the spurious "<>" disappears. It seems to me that this is a bug. It was not present in 8.2x.
The main-content html is put into the page even though this block does not appear in the Block Layout page