e collapsed with its parent's top margin, the top border edge of the box is defined to be the same as the parent's.

The top border edge of <body> is below its top margin, which is collapsed with that of #b.
-->
<style>
html, body {
    margin: 0;
}
div {
    width: 32px;
    height: 32px;
}
#a {
    float: left;
    background: green;
}
#b {
    background: blue;
    margin-top: 32px;
}
</style>
<div id=a></div>
<div id=b></div>

