How do I center a <div> vertically?

How do I center a <div> vertically?

I have managed to horizontally align a Div object. Here is how I did it. I got the width and the height of the object I wanted to center (XAML CODE)

HTML Code:

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<Grid  Background="White" Height="300" Width="738" VerticalAlignment="Center" HorizontalAlignment="Center">

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Then, in the html file that hosts the silverlight control I did this:

HTML Code:

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<div id="silverlightControlHost" style="width:738px; height: 300px;  margin-top:auto; margin-bottom:auto;  margin-left: auto; text-align:center; margin-right: auto;">

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That puts the control centered horizontally at the top of the web page.

Now, how do I center it vertically?