Status: draft proposal, not for deployment
In Understanding Progressive Enhancement Aaron Gustafson uses the analogy of a peanut M&M to illustrate how the content, presentation, and behaviour layers of a web page interact:
Reproduced from the A List Apart article:
http://www.alistapart.com/articles/understandingprogressiveenhancement
This is a useful analogy because it puts content at the heart of design decisions.
Some people prefer plain peanuts to M&Ms; some users will access your pages without the presentation and behavioural layers! Knowing this, your pages should be usable 'naked' - with no style information at all.
You can view this page without its presentational layer by clicking here. [Reload the page to get the pretty view back!]