Progressive enhancement

http://en.wikipedia.org/wiki/Progressive_enhancement

Progressive enhancement is a strategy for web design that emphasizes accessibility, semantic markup, and external stylesheet and scripting technologies. Progressive enhancement uses web technologies in a layered fashion that allows everyone to access the basic content and functionality of a web page, using any browser or Internet connection, while also providing those with better bandwidth or more advanced browser software an enhanced version of the page.

History
"Progressive Enhancement" was coined by Steven Champeon of hesketh.com in a series of articles and presentations for Webmonkey and the Sxsw Interactive conference between March and June of 2003

Introduction and background
The strategy is an attempt to subvert the traditional 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 practice, graceful degradation has been supplanted by an attitude that the end user should "just upgrade". This attitude is due to time and budget constraints, limited access to testing alternate browser software, as well as the widespread belief that "browsers are free". Unfortunately, upgrading is often not possible due to IT department policies, older hardware, and other reasons. The "just upgrade" attitude also ignores deliberate user choices and the existence of a variety of browser platforms; many of which run on handhelds or in other contexts where available bandwidth is restricted, or where support for sound or color and limited screen size, for instance, are far different from the typical graphical desktop browser.

In Progressive Enhancement (PE) 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 or JavaScript (or other advanced technologies, such as Flash or Java applets or SVG, etc.) All such enhancements are externally linked, in order to avoid forcing browsers of lesser capability to "eat" data they do not understand and cannot handle, or which would swamp their Internet connection.

Core principles
- basic content should be accessible to all browsers
- basic functionality should be accessible to all 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 browser preferences are respected

Benefits for accessibility
Web pages created according to the principles of PE 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 PE sites work with older tools designed to deal with table layouts, "tag soup," and the like.

Benefits for search engine optimization (SEO)
Improved results with respect to Search Engine Optimization is another side effect of a PE-based Web design strategy. Because the basic content is always accessible to search engine spiders, page built with PE methods avoid problems that may hinder search engine indexing