I'm not sure I fully understand your question. You can reduce the opacity of the object, but if you export it on a solid colored background, it will appear opaque on your web page. If what you want is to export a semi tranparent object, then change the canvas to transparent, reduce the opacity of the object, and export it as a 32 bit PNG with Alpha transparency and no matte color. Depending on the complexity of the object, you might be able to export it as an 8 bit PNG with alpha transparency.
to do this, you need the Extension "Export Selection", which you can find here:
With this, you can select an Object and export it directly with transparent background, even if it sits on colored background.