Welcome to WebmasterWorld Guest from 18.104.22.168
Forum Moderators: not2easy
While there is nothing visibly over "a", from the browsers point of view there is something over div a, namely div b. While the background is transparent and you can see the contents of div a, div b (an object) is still sitting on top of div a.
Therefore, when your mouse is trying to select the text in div a, it is in fact just trying trying to select anything in the top most object, ie div b.