<!doctype html>
<style>
.claz {
	margin: 1em 2em 1em;
}
.one {
	margin: 0 auto;
        width: 10em;
}
#a {
	margin: 1em;
}
</style>
<div style="width: 1000px">
spacer
<div class=claz>left, right: 2em<br>top, bottom: 1em.</div>
spacer
<div class="clazz one">width: 10em<br>left, right: auto<br>top, bottom: 0</div>
</div>
