Javascript, CSS, SVG, Flash and other technologies (even simple bit-map images) can be applied to a page to enhance the visitor's experience if her browser is capable of those technologies. They should augment the basic HTML of the page, but some sites depend too much on them — if a visitor doesn't have a particular capability, or has turned it off, the page becomes unusable or useless.

Javascript is the usual suspect. This allows the page author to program the visitor's browser. It works best when applied to multiple steps of an interaction between the visitor and the site, e.g., when filling in a sequence of forms, as it can locally sanity-check inputs, and enable or disable form fields that depend on the values of other fields, as a time-consuming check with the server can be avoided.

However, it is no substitute for server-side checks, as Javascript may be disabled by the visitor. Similarly, links should not depend on javascript: URIs or use onMouseOver attributes without functional hrefs as back-up.

• Javascript form validation - doing it right
  Stephen Poley's notes on JavaScript with forms
• Web Matters - What - no Javascript?
  Stephen Poley's reasons for disabling JavaScript