If you're using WP 8.1 then just tap and hold back key (left to windows logo) and swipe down to close.
If you're using WP 8 then press X (at top right corner) instead of swiping down.
There is no X in the top right corner of the opened website with Win8. It appears the only way that I could find is to click on the back button and then close it on the X.
There is no X in the top right corner of the opened website with Win8. It appears the only way that I could find is to click on the back button and then close it on the X.