Should <div style="margin-right:10px;"> be working as well?
Yes (and No if you don't understand the DIV tag).
The <div> will fill the width of the screen with a 10px space on the right edge.
Add this style and you will see the effect. It will add a black border around your DIV
<div style="margin-right:10px; border:1px solid #000">
margin:10px will add a 10px space all around.
you can also use.. padding:10px which will add a 10px padding inside the DIV border.
Read up on using CCS and you will find all sorts of effects
POST EDITED: see comment below