The background color (Stage color) of a SWF file can be set to transparent. The background color or image of the HTML page that contains the SWF file shows through. This technique allows layering of SWF content with DHTML (Dynamic HTML) content.
Not every web browser handles transparency in the same way. Be sure to test your SWF file in all browsers that you want to enable your audience to use. Most Linux browsers do not support Flash transparency. This table lists several browsers that support transparency.