Enhanced Tiles should have some sort of meta tag for the hovered and (maybe) unhovered states. This way, website owners can have their own "enhanced tiles", if they wish, and it can be another way websites can seem more integrated with the browser. If there is no meta, it will simply fall back to the preview. :) The browser also wouldn't grab the meta from another page ex. if the meta is defined on the homepage, and they pin a not-homepage page, then it would not show it, and vice versa.