Frankly, in my experience, there's not a lot you can do very successfully, if the designers have failed to mark up content with proper semantics, or have applied counter-productive styles. Here are some things I've tried, with limited success. They can be used in combination.

• Configure your browser and/or edit your user stylesheet for optimum display of unstyled documents. Find a document that uses no stylesheets, and adjust your browser's settings so that you are happy with it. Choose a font size that is comfortable on your eyes.

  If you customarily use a very wide window, and you don't like the unstyled text because it's difficult for your eyes to locate the start of one line after reading the previous, either increase the font size, or apply body { max-width: 35em } (or similar) in your user stylesheet (without !important, so sites can override if necessary). If you can't do that, you might reconsider why you are using such a wide window – you are basically inviting a page to fill it, so you can't complain if it does.

  • Firefox Help: Editing Configuration Files — userContent.css
    Your user stylesheet in Firefox
  • Stylish | userstyles.org
    An easier way to edit your user stylesheet in Firefox

• Edit your user stylesheet for particular sites. Some browsers support an internal extension to CSS, allowing user styles to apply to particular sites or domains. This can be tricky though, as it can depend on a little foresight from the site designer, if the fixes you need to apply are complex, critically depending on expressing where and when to apply them within the page.

  • CSS:@-moz-document - MDC
    How to apply user styles to specific sites

• Disable styles for certain sites. Browsers or browser extensions might be able to remember misfeatures of each site that you don't like, such as appalling CSS. Hopefully, the designer has had enough foresight to make the page work without CSS (for the sake of at least those who don't have it), and then you can exploit that to by-pass their muck.

  • Style Sheet Chooser II
    A Firefox extension that remembers your choice of style per site
  • StyleSheet Chooser Plus
    Another Firefox extension doing the same thing

• Use client-side scripts to edit bad HTML and CSS automatically, e.g., with Greasemonkey. These could remove width attributes from <table> elements, hide spacer images (which are often very long and thin, or simply very small), remove absolute positioning, or complete or remove incomplete colour specifications.

  • Greasemonkey
    A Firefox extension that applies user-defined JavaScript code to the pages you visit