wmode is a parameter that excludes only the <embed> that refers to Flash movies. The default value is wmode=window .
wmode=window
When wmode=window , the Flash movie is not displayed on the page. It is displayed in a separate window than the contents of the browser (as verified using Spy ++ or WinSpy ++ ). This mode will have better performance, since the browser does not have to redraw part of the page on each frame. However, this mode prevents you from showing content above or below the Flash movie.
- Best performance
- It is displayed in a separate window.
- Opaque background
- Does not allow content below
- Does not allow content above
wmode=opaque
When wmode=opaque , the Flash movie is displayed as part of the page. Window not created for movie. The movie will be displayed in the background color set during the publishing process, and the content will not be available. On each frame, the content that appears above the movie must be redrawn by the browser, which affects performance.
- Good performance
- Presented as part of the page.
- Opaque background
- Does not allow content below
- Allows content above
wmode=transparent
When wmode=transparent , the Flash movie is displayed as part of the page. Window not created for movie. The background color of the film will be transparent. Thus, any opaque part of the film will allow the contents of the main content to be displayed. On each frame, the content that appears above and below the film must be redrawn by the browser, which significantly affects performance.
- Fair performance
- Presented as part of the page.
- Transparent background
- Allows content below
- Allows content above
EDIT: Below are answers to your additional questions ...
Is wmode attribute of only FLASH?
Yes, wmode is only available in <embed> tags by inserting a Flash movie.
What are the performance implications between different values?
wmode=window will have better performance since the Flash movie will completely separate from the page itself. The browser does not need to update or calculate the position of the z content displayed on top of the Flash movie, because the Flash movie in this mode is displayed in a completely separate window (as you can check with Spy ++).
wmode=opaque and mode=transparent both follow very similar rendering paths. However, they are slower than wmode=window , because the browser must check the elements to see if they display higher than the movie, and displays them on every frame. Note that wmode=transparent slower than wmode=opaque , as it should also display the underlying content as well as overlay the content.
So, in order of performance ...
FASTEST ----------------------------- SLOWEST WINDOW OPAQUE TRANSPARENT