Everyone has JavaScript, right?
kryogenix.org/code/browser/everyonehasjs.html
Stuart Langridge presents this simple, yet convincing flow chart to illustrates all the various things that may go wrong as users request a web site requiring JavaScript code.
As with technology use vs. non-use, working JavaScript in a browser is not a binary dichotomy (script support on vs. off), but a fluid scale with various degrees of support. A good designer and developer takes this reality into account and builds good experiences immune to such aspects.
For some evidence in form of hard numbers:
How many people are missing out on JavaScript enhancement?
gds.blog.gov.uk
