But the Kindle is a different beast, it does not read ePUB, but Mobipocket, so whatever the ePUB says is meaningless, all that counts is what goes into the Mobipocket. You are using some converter, that converter might add something or not, it might override the default or not... If you were creating the Mobipocket directly, I'd tell you to use:
Code:
<h2 height="0em">2</h2>
since in Mobipocket pseudo-HTML, "height" means "space above".