Nobody doing business on the Web can honestly say, "accessibility isn't my department." Every stakeholder in a web project stands to benefit from and is empowered to contribute to an accessible product. Fortunately for us, we can learn from our past successes and failures to build a better web together.
"Accessibility" refers to the usability of a website by people across a diverse range of abilities and disabilities. From myopia to arthritis to dyslexia, each user's experience of your content will be different. I couldn't put it better than the W3C:
The principles of accessibility require us to remove—or at least minimize those barriers as much as possible. Unfortunately, the state of accessibility on the web is very bad. It's a big problem that directly impacts millions of people. Despite this, I'm optimistic about the future. Internet technology has come a long way in the last 30 years, and in that time has closed significant accessibility gaps.
Looking to the past
The browser wars and their aftermath resulted in a fractured and frustrating web. Cross-browser compatibility seemed impossible at first. Over time, as technology developed and understanding grew, the problem became expensive but manageable. These days, compatibility is a given. It's expected that new sites are built to work on all major browsers, even across an array of devices. No modern web agency would pitch a client a design built to work only on one specific desktop browser.
The modern practice of implementing cross-browser compatibility is called "progressive enhancement.” Progressive enhancement means that access to the core content of your site should not be tightly coupled to presentational specifics, which might vary from device to device. Instead, the content should be available in a universally accessible format and stylistic enhancements should be progressively applied such that they don't interfere with less-capable devices. The appearance should be informed by the content and affordances, not the other way around. You first must answer, "What does this site say and do?" before asking, "How does this site look?" It's important to make these distinctions because there's a whole universe of web-connected devices with different capabilities. From big-screen TVs to hand-held devices to screen readers, the appearance of your site will vary widely. If you've followed the principles of progressive enhancement, this won't be a hindrance; the message your site conveys and the opportunities it affords will be universally accessible.
Over on quirksmode.org, Peter-Paul Koch recently posed the following question: “What is the exact relationship between progressive enhancement and accessibility? I think that they are, essentially, the same process. Accessibility aims to optimize an experience across a spectrum of user capabilities. PE aims to optimize an experience across a spectrum of user agent capabilities. We tend to think of user agents as browsers and devices, but the term could easily apply to eyes, ears, and fingers. Isn't a user's physical body also their agent? Put another way; progressive enhancement is accessibility for browsers.”
What is the application of color to a website if not a progressive enhancement targeting users that can discern colors? Any number of technological or physiological mechanisms may prevent a user from seeing the full-color spectrum. Whether we call it a "disability" or an "incompatibility," the principles of both PE and accessibility require us to consider such users.
Conversely, isn't the inability to process javascript essentially a disability of the browser? We would probably call handling this case a PE concern, but what if the user has disabled javascript in order to mitigate a physical disability? The user agent’s capabilities have been modified to better match the capabilities of the user, and now it starts to look like an accessibility issue.
Looking to the future
Compared to the vast diversity of human abilities and disabilities, the differences between browsers are really quite modest. Devices are produced en masse and software is updated in discrete revisions. That means the practice of progressive enhancement tends to be quantitative, lending itself to compatibility tables and polyfills. By contrast, user disabilities don't usually come with version numbers and changelogs; each user's experience is singular. Accessibility is therefore approached more qualitatively; guidelines and principles are usually the authoritative accessibility sources. Even the best-automated accessibility testing tools provide incomplete data because the problem space is so much larger.
As more browsers and devices are released and the technology and best practices for accessibility improve, I anticipate that progressive enhancement will become more qualitative while accessibility becomes more quantitative. Look how far we've come in 30 years of building websites. We already see assistive devices, essentially "accessibility polyfills," like text-to-speech and keyboard navigation becoming more sophisticated and cheaper. Where will we be in another 30 years? Or 60?
The movie Freaky Friday depicts a mechanism by which humans can switch bodies. Consider the accessibility implications if such a technology could be perfected. On one hand, any disabled user could download themselves into an able-bodied 20-year-old and bypass any accessibility hurdle. On the other hand, one can imagine notices like, "This website is best viewed in a body with 20/20 vision and excellent dexterity."
The Matrix takes things a step further, showing us people who can plug computers directly into their brains. This arrangement bypasses the user's body entirely, the ultimate accessibility polyfill! At this point, user agents, accessibility, and progressive enhancement are all moot.
Unfortunately, the modern user who's relying on assistive technologies does not have such an opportunity. They're stuck with the best we can do now, which, it turns out, is actually quite a lot (if we heed the advice of accessibility experts). Let's do our best to eliminate barriers between our users and our content. Because accessibility concerns begin with the content itself and trickle down through all the layers of technology used to deliver it, everyone involved in a web project, from content editors to programmers to project managers to sales (especially sales!), should commit to building an accessible experience. Whether we call it progressive enhancement or accessibility, our goals are the same: to make the content available to the widest audience possible.