Some people like use chars and html-codes as a default content for buttons and links. The idea is to put some simple character or code which will look like an icon, and later, if necessary, replace it with a real icon at the css-level. A "close" button is a pretty common case and people usually use the "X" character in there. The problem with it is that it has wrong dimensions and doesn't look like a normal cross on a close button. Luckily there is a better way. There is a standard html code for the mathematical multiplication symbol ×.
It looks like that
As you see it has the correct shape of the close symbol, and it still a normal character which you can paint with css just as you like, use hover effects, etc.