Over the last two years Javascript has gained in popularity and taken a large market share away from Adobe Flash in delivering online multimedia and interactive applications. I believe the main reasons for this are developers becoming more concerned with accessibility and simplification of implementing Javascript.
On the first issue I am unconvinced there is a winner. Though Flash can be made in an accessible manner supporting tab and access keys it isn't a suitable tool for delivering a universally usable website or widget. Flash is mainly used for creating interactions that simply do not suit this kind of navigation, or at least developing a Flash project supporting keyboard focus and mouse would take too much time for the small audience gain it may or may not receive. Conversely, forcing the majority of mouse users on to the keyboard to interact with your website is also highly likely to confuse rather than captivate the audience.
As good or bad as Flash may be for accessibility this does not grant my automatic support for Javascript. As most good designers and developers know, a website relying on Javascript for core functions must be built using progressive enhancement. The common adoption of Javascript frameworks has meant there are thousands of plug-ins, widgets and mash-ups in widespread use that lack of any degradation - though this may be relief for users who do not want to know what you're writing on Twitter anyway.
OK, this is all a bit pre-2004 and Javascript access is normal for over 99% of Internet users but there is a much more important question: 'But what does it look like in Internet Explorer/Opera/Safari/Chrome and on my mobile phone/console/MP3 player?'. Though many JS frameworks are well tested and provide a shortcut for providing cross-browser compatible code there are very few areas where behaviour truly is, especially when mobile. Flash entirely side-steps this, the player is available on almost anything and will render and work in the same way almost all of the time. Flash also negates the need to worry about end users having support for various media other than a version of Flash Player installed equal or above the one you have published your work with.
I do not think there is a winner when accessibility is primary concern. Creating accessible Javascript or Flash is difficult, though the former requires full understanding of many programming languages and how they all work together; HTML, CSS, Javascript, browser compatibility and at least the basics of a dynamic server-side language. Flash however requires an understanding of only one.
