I've placed an area as an overlay effect over another coloured area. When I upload the document, the effect is not shown on the web. Is this effect not supported by "Publish Online"? Thanks.
You’re going to have provide way more detail on what you mean by an overlay effect as well as some screenshots.
If by overlay you are referring to the overlays panel, that is only good for DPS or AEM Mobile and has no use in any other output.
All interactivity from the Overlays panel is only meant for DPS or AEM as it is now called, so it will not work in Publish Online or FXL ePub.