Centering Figures in Html With CSS: No Luck!

I have been able to center my figures in pdf successfully using Bob
Stayton's advice in his book.  However, I can't yet get my css style
sheet to center the html output.

I have my figures in DocBook in a chunk like this:

<figure>
  <title>A graphic</title>
  <mediaobject>
    <imageobject role='html'>
      <imagedata
          width='4.5in'
          fileref="./images/dmPlot.png"/>
    </imageobject>
    <imageobject role='fo'>
      <imagedata
          width='4.5in'
          fileref="./images/dmPlot.png"/>
    </imageobject>
  </mediaobject>
</figure>

In my css file I have tried these:

div.mediaobject {
  align: center;
}
div.mediaobject {
  text-align: center;
}

Neither work.  But if I add in my fo file in the mediaobject block
attributes "align='center'", the figure centers nicely.

I have tried to find where DocBook does the translation from css to fo
so I can intervene ("customize") but haven't had much luck.

What am I doing wrong?

Thanks.

-Tom

Thomas M. Browder, Jr.
Niceville, Florida
USA