Let me begin by saying that Adobe Flash is very far from falling out of use. Conservative estimates say 95% of Web users have Adobe Flash Player installed. The number of websites that employ Flash is harder to estimate, but most would put the number between 25% and 50%. And as this website so succinctly indicates, Flash is still alive and well.

All that being said, I do believe that the use of Flash on the Web is about to decline, if it hasn’t already (again, very difficult to estimate). Here are a few reasons why I believe the presence and importance of Flash on the Web will decline over the next several years:

Last Wednesday, the Apple iPad joined the growing list of devices that do not support Flash. The long list of mobile devices that do not display Flash should concern Adobe. If people are forced to choose between the mobile Web and Flash, mobile will win every time.

The use of Flash, especially in 100% Flash sites, can have a limiting effect on a site’s success in search engines. Many people will try to convince you that search engines can handle Flash without an issue, and in fact, search engines have made great strides in “reading” Flash files. But Google and others engines were built to crawl Web text. If Flash animations limit the amount of text that appears on your webpages, I think that you may be hurting yourself from an SEO perspective.

Flash is prohibitive to regular site updates, especially if site administrators are not technically savvy. Static content is growing increasingly less acceptable on the Web. Users now expect regularly updated content on the sites they visit, and a static site limits how you can engage your visitors. Compared to text, or even HTML, Flash files are difficult to update, and will limit what site owners can do on their own site.

HTML5 is definitely a hot word on the Web right now. This next standard for HTML may not be fully accepted and standardized for years, but already it is promising to step on the toes of Flash with its increased emphasis on embedded media. YouTube, for example, is now offering some videos in HTML5 format in addition to the traditional Flash. As another example, Google recently circumvented Apple’s ban of Google Voice in the App Store by building an online HTML5 application that iPhone users could access through the Web. As mentioned, the iPhone does not support Flash, so the choice of HTML5 was a critical one.

Flash doesn’t fit into the W3C’s plans. The World Wide Web Consortium was founded in 1994 by Internet pioneer Tim Berners-Lee to maintain standard practices for Web publishing. This organization commands quite a bit of respect, and was instrumental in instituting the separation of content, design, and functionality (HTML, CSS, and JavaScript) on the Web. Flash doesn’t fit very neatly into this model, and you have to jump through a few hoops to insert Flash into a web page and maintain its W3C validity.

Adobe Flash stands in direct opposition to the open source model, which is gaining momentum on the Web. Open source has developed a strong following, and it seems like it will continue to gain force for the foreseeable future. Flash, as a proprietary, paid software, simply does not fit into the philosophy.

Flash violates traditional usability rules. As the Web gets more competitive, usability is the means to stand out from the crowd. And an important part of usability is obeying accepted standards of use. For example, Web users, and computer users in general, are accustomed to using the right-click menu for certain functions. Flash does not allow this. People also rely on navigating with the Back button of the browser, which is not possible in full-Flash sites.

In conclusion, I didn’t mean for this post to be a rant against Flash, or a directive never to use it on the Web. Rather, I simply see all of these factors predicting the decline of Flash’s use in the coming years. Only time will tell if I am right or wrong.