Developers can set the Window Mode (wmode) property of the SWF content to control layering and transparency of the content in the browser. Regardless of the wmode value, Flash Player displays content viewed in full screen mode using direct path rendering. Using direct path rendering enables hardware accelerated presentation capabilities such as Stage Video and Stage 3D.
To deliver the highest performance playback of Flash-enabled content in the browser, usewmode=direct. This attribute enables hardware accelerated presentation capabilities, including Stage Video and Stage 3D. Avoid overlapping the SWF content with HTML elements (for example, HTML-based pop-up menus) intended to appear above the SWF file. Explicit layering control is only supported with some modern browsers (see table below). In other browsers, the SWF content always appears above other HTML elements.
If you require explicit layering control, transparency, or HTML elements that float above SWF content, use wmode=opaque or wmode=transparent. You can then control layering in relationship to other elements on the page through HTML. However, these "windowless" modes reduce playback performance compared to wmode=window (the default) and wmode=direct.