Dynamic Flash

The Flash Player has supported the wmode parameter since version 3 or so, but I didn’t realise until recently that setting wmode to ‘opaque’ or ‘transparent’ causes the Flash movie to be completely invisible to assistive technologies such as screen readers.

The default value for wmode is ‘window’, which causes the Flash movie to be rendered in its own invisible window floating above all the content in the browser window. This causes problems with DHTML menus and the like, and the common solution offered is to set wmode to ‘opaque’, instructing the Flash Player to render its content directly onto the browser’s window. This means that CSS styles such as z-index are respected, allowing you to place DHTML content over the top of the Flash movie. Perfect, right? Apparently not.

The first drawback, and one that’s been known for a while, is that performance degrades when wmode is set to ‘opaque’ and even more so with ‘transparent.’ However, a bigger concern is that if wmode is set to anything other than ‘window’ the Flash movie will be completely inaccessible to screen readers, undoing any good work you may have done with the accessibility properties.

Of course, this can have its uses. If your Flash movie is purely a visual thing and doesn’t actually contain any content, you can use the wmode parameter to hide it completely from screen readers. Just be sure that’s what you want to do.

For more information on wmode, see the How to make a Flash movie with a transparent background TechNote. For more information on how screen readers interact with Flash movies, see Andrew Kirkpatrick’s excellent ‘In search of… a perfect plugin technique’ article

Update: I’ve updated the article to reflect the additional information and clarification provided by John Dowdell - see comments.