Transparency through Opera
Transparency through Opera can be used to create effects where a browser window overlays other graphics layer in the hardware. To put it in another way, it is possible to get alpha levels (below 100% solid) on the final Opera buffers by using Web technologies.
One of the big applications of this technology is to put HTML-based overlays on top of streaming media. This is illustrated in the image below:

The weather widget in the top right corner is interactive and based on Web technologies. The movie playing in the background is drawn in a separate graphics layer and has nothing to do with the browser engine. Transparency needs to be implemented in both the browser application and in the Web elements on the page displayed in the browser. Enabling transparency will causes a small overhead so this option is usually not enabled unless needed.
Premultiplied alpha
Most packages comes with premultiplied alpha enabled which needs to be addressed when alpha blending. With premultiplied alpha enabled, the R,G and B parts of the pixel data are multiplied with the alpha value. This will make alpha blending faster as some multiplication steps are not needed when blending. For more info about alpha blending and premultiplied alpha, please see the Wikipedia article on Alpha compositing. Notice that alpha_front will be between 0 and 255 and that alpha_back is always 255 in this implementation. This gives the following formula:
pixel_result = pixel_front + pixel_back * (maximum_alpha - alpha_front) / maximum_alpha