There`s some extra space under textarea tag. From 1 to 4 pixels in different browsers. The markup is very simple:
In my case, thirtydot's answer didn't work well with the parent <div>'s bottom border.
display: block suited me nicely though.
Add vertical-align: top to textarea.
The reason for the gap is that textarea is an inline (or inline-block) element, and the gap is the space reserved for descenders in text. I don't know exactly why the gap is different between different browsers.