I just spent a lot of time looking into this. I've used Photoshop to copy semi-transparent textures for some time while prototyping (with the destination of Axure RP).
Now, I've tried using the new clipboard API in html 5. I dissected the PNG files that are generated by this process.
If I use the clipboard (with a rectangular marquee) to copy a semi-transparent texture to the browser with the clipboard API, it sets the alpha byte to FF (completely opaque).
If I use the drag and drop API to copy a "Save as Web" PNG 24 file to the browser, it works as expected and transparent pixels work (and the alpha byte is not forced to FF).
Note: Even thought Photoshop says it's 24 bit, it actually exports it as 8 bit still (confirmed).
.pngfile or its bitmap while it is opened in an editor/viewer? If it is the file, then Photoshop (should) be able to open the file correctly, transparency and all. If it is the picture’s actual pixels, then it depends on the source program and whether it is putting the transparency data in the clipboard or not. – Synetech Jul 7 '12 at 20:34