The most compatible way I have found for centering something in the browser is to place the visible HTML inside a single table like this:
<table height="100%" width="100%"><tr><td align="center" valign="middle">
... all my centered stuff goes here ...
</td></tr></table>
Most likely there is a more compliant CSS method for doing this, but I'll leave that up to someone more familiar with CSS.
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Peter