Unfortunately this is not a PowerPac issue. Opacity from author mode is set for HTML 5 browsers like Chrome and Firefox during the export process. For some reason, when you export your page, the opacity setting is lost. I can confirm that opacity is unset for the yellow field when I use Firebug to examine the HTML in your export. When I export the page, the opacity property is present and set at 0.5 which the right behavior.
You should be able to create the same page and disable the PowerPac altogether. Then export the page and the transparency should be present in all browsers. Of course, HTML 5 support will be disabled in IE, but you can try this for testing purposes.
Here is my Firefox export screen shot with PowerPac disabled:

- Firefox screenshot with PowerPac disabled. Note that transparency is preserved.
- Image 3.png (2.77 KiB) Viewed 1089 times