Background behind rounded corners is not transparent it is white

I have tried everything I can think of a failed.

I want to export a rectangle with rounded corners with a colour fill linear gradient.

This is to create a background url for a button which which need to emulate standards compliant CSS in Internet Explorer.

I am sure many many people have to do this IE hack.

But what ever I do the background under the rounded corners shows up as white space which is no good.

I have:

Made sure the image is sized to the nearest whole pixel.
Made sure it is snapped to grid with whole pixel positioning
Made sure I have set clipview on.
Made sure I have transparent background set.

I have tried the export as a PNG, GIF, JPG and SVG.

None work.

Is this possible or am I wasting my time?

Try exporting as PNG and choose the Colour depth as 'True colour + Alpha'

Please see the attached image, and the chosen option, and the left had part is showing the rounded corners do not have any white corners.

You can also export a 256 color PNG. Click on the wine glass icon to make the background transparent.

If the image is going to appear over a dark background, or any background other then white, place a larger rectangle under the button shape that is the same or close to the color over which the button will appear. When you export, select the button but do not select the large rectangle.

JPEG images do not support transparency, so PNG either True Color + Alpha or 256 color with transparency is your best bet.