Progressive enhancement is a strategy for web design that emphasizes core webpage content first. This strategy then progressively adds more nuanced and technically rigorous layers of presentation and features on top of the content as the end-user's browser/internet connection allow. The proposed benefits of this strategy are that it allows everyone to access the basic content and functionality of a web page, using any browser or Internet connection, while also providing an enhanced version of the page to those with more advanced browser software or greater bandwidth.
History
"Progressive Enhancement" was coined by Steven Champeon & Nick Finck at the SXSW Interactive conference on March 11, 2003 in Austin, and through a series of articles for Webmonkey which were published between March and June 2003. Specific Cascading Style Sheets techniques pertaining to flexibility of the page layout accommodating different screen resolutions is the concept associated with responsive web design approach..net Magazine chose Progressive Enhancement as #1 on its list of Top Web Design Trends for 2012. Google has encouraged the adoption of progressive enhancement to help "our systems see usable content and basic functionality when certain web design features are not yet supported".
Introduction and background
The strategy is an evolution of a previous web design strategy known as graceful degradation, wherein designers would create Web pages for the latest browsers that would also work well in older versions of browser software. Graceful degradation was supposed to allow the page to "degrade", or remain presentable even if certain technologies assumed by the design were not present, without being jarring to the user of such older software. In progressive enhancement the strategy is deliberately reversed: a basic markup document is created, geared towards the lowest common denominator of browser software functionality, and then the designer adds in functionality or enhancements to the presentation and behavior of the page, using modern technologies such as Cascading Style Sheets, Scalable Vector Graphics, or JavaScript. All such enhancements are externally linked, preventing data unusable by certain browsers from being unnecessarily downloaded. The progressive enhancement approach is derived from Champeon's early experience with Standard Generalized Markup Language, before working with HTML or any Web presentation languages, as well as from later experiences working with CSS to work around browser bugs. In those early SGML contexts, semantic markup was of key importance, whereas presentation was nearly always considered separately, rather than being embedded in the markup itself. This concept is variously referred to in markup circles as the rule of separation of presentation and content, separation of content and style, or of separation of semantics and presentation. As the Web evolved in the mid-nineties, but before CSS was introduced and widely supported, this cardinal rule of SGML was repeatedly violated by HTML's extenders. As a result, web designers were forced to adopt new, disruptive technologies and tags in order to remain relevant. With a nod to graceful degradation, in recognition that not everyone had the latest browser, many began to simply adopt design practices and technologies only supported in the most recent and perhaps the single previous major browser releases. For several years, much of the Web simply did not work in anything but the most recent, most popular browsers. This remained true until the rise and widespread adoption of and support for CSS, as well as many populist, grassroots educational efforts showing Web designers how to use CSS for layout purposes. Progressive enhancement is based on a recognition that the core assumption behind "graceful degradation" — that browsers always got faster and more powerful — was proving itself false with the rise of handheld and PDA devices with low-functionality browsers and serious bandwidth constraints. In addition, the rapid evolution of HTML and related technologies in the early days of the Web has slowed, and very old browsers have become obsolete, freeing designers to use powerful technologies such as CSS to manage all presentation tasks and JavaScript to enhance complex client-side behavior. First proposed as a somewhat less unwieldy catchall phrase to describe the delicate art of "separating document structure and contents from semantics, presentation, and behavior", and based on the then-common use of CSS hacks to work around rendering bugs in specific browsers, the progressive enhancement strategy has taken on a life of its own as new designers have embraced the idea and extended and revised the approach.
Core principles
The progressive enhancement strategy consists of the following core principles:
Basic content should be accessible to all web browsers
Basic functionality should be accessible to all web browsers
Sparse, semantic markup contains all content
Enhanced layout is provided by externally linked CSS
Enhanced behavior is provided by unobtrusive, externally linked JavaScript
End-user web browser preferences are respected
Support and adoption
In August 2003 Jim Wilkinson created a progressive enhancement wiki page to collect some tricks and tips and to explain the overall strategy.
Designers such as Jeremy Keith have shown how the approach can be used harmoniously with still other approaches to modern web design to provide flexible, but powerful, user experiences.
Aaron Gustafson wrote a series covering the fundamentals of progressive enhancement, from the underlying philosophy to CSS approaches to how to handle JavaScript, for A List Apart.
Others, including Dave Shea, have helped to spread the adoption of the term to refer to CSS-based design strategies.
Organizations such as the Web Standards Project, which was behind the creation of Acid2 and Acid3 tests, have embraced progressive enhancement as a basis for their educational efforts.
In 2006 Nate Koechley at Yahoo! made extensive reference to progressive enhancement in his own approach to Web design and browser support, Graded Browser Support.
Steve Chipman at AOL has referred to progressive enhancement as a basis for his Web design strategy.
David Artz, leader of the AOL Optimization team, developed a suite of Accessible Rendering Technologies, and invented a technique for disassembly of the "enhancement" on the fly, saving the user's preference.
Chris Heilmann discussed the importance of targeted delivery of CSS so that each browser only gets the content it can handle.
Scott Jehl of Filament Group proposed a "Test-Driven Progressive Enhancement", recommending to test the device capabilities before providing enhancements.
Wt is an open-source server-side web application framework which transparently implements progressive enhancement during its bootstrap, progressing from plain HTML to full Ajax.
Benefits for accessibility
Web pages created according to the principles of progressive enhancement are by their nature more accessible, because the strategy demands that basic content always be available, not obstructed by commonly unsupported or easily disabled scripting. Additionally, the sparse markup principle makes it easier for tools that read content aloud to find that content. It is unclear as to how well progressive enhancement sites work with older tools designed to deal with table layouts, "tag soup", and the like.
Improved results with respect to search engine optimization is another side effect of a progressive enhancement-based Web design strategy. Because the basic content is always accessible to search engine spiders, pages built with progressive enhancement methods avoid problems that may hinder search engine indexing.
Criticism and responses
Some skeptics, such as Garret Dimon, have expressed their concern that progressive enhancement is not workable in situations that rely heavily on JavaScript to achieve certain user interface presentations or behaviors, to which unobtrusive JavaScript is one response. Others have countered with the point that informational pages should be coded using progressive enhancement in order to be indexed by spiders, and that even Flash-heavy pages should be coded using progressive enhancement. In a related area, many have expressed their doubts concerning the principle of the separation of content and presentation in absolute terms, pushing instead for a realistic recognition that the two are inextricably linked.
