Serving old browsers limited JavaScript and CSS

The Guardian website viewed with Internet Explorer 8: a very basic document with little to no CSS applied.
The Guardian navigation as seen in Internet Explorer 8. Unsophisticated yet functional. as viewed with Internet Explorer 9: a very simple layout without much CSS applied.
The homepage as seen in Internet Explorer 9.

Since we’re still stuck with a small percentage of users still on various versions of Internet Explorer and other older browsers, a good way to deal with those seems to be to only serve most or all of our JavaScript and CSS to browsers that cut the mustard, leaving the older set with a basic but functional experience, without risk that our newer, shiny stuff will inevitably break, or the need for polyfills that may or may not work.

See also: Cutting the mustard with only CSS