How to Make a Swf with a Transparent Background (In Flash Professional)
So, you want to make a swf with a transparent background, eh? Well, this is actually very easy to do, but it can be a little tricky finding out where Adobe put this option. First, open flash professional CS6 (this will also work for Flash CC and most older versions of flash professional). Go to File -> Publish Settings. On the left side of the settings dialog box under "other formats "click on HTML wrapper (you do not need to have this checked in order for it to work).
Clicking on the words HTML wrapper should change the content of the rest of the dialog window. Towards the bottom you'll see an option for window mode. In the drop-down select transparent windowless and hit OK.
Now that's it! When you drop your swf in a webpage it should show the content of the page through the swift, and if you are writing your own HTML to embed the swf be sure to set wmode equal to transparent. Happy developing. - Jim