The HTML that is exported by GlowScript is merely a <div>...</div> tag with the GlowScript for the simulation between the div tags.
You can put this into a html page within center tags. However, there is also another way. See the example at:
In this case, I put the GlowScript in a separate html page. I then embedded this html page using <iframe> tags such as:
<center>
<iframe src="./Chapter_3-4/celestial_sphere/ecliptic-gs.html" width=500 height=500 align="center"></iframe>
</center>
The source of the simulation is posted at:
As you can see, I centered the iframe.
Aaron