How to create Internet Explorer Drupal 7 Conditional Stylesheets for IE CSS

To create Conditional Style sheets for Internet Explorer for a Drupal 7 site you need to use the Conditional Stylesheet Module.
Download and install and enable the module. There is no module configuration - when the module is enabled you add the stylesheet information to the theme .info file.

sites/all/themes/yourtheme/yourtheme.info

name = ThemePlanet
description = A really cool theme to do stuff in
core = 7.x
engine = phptemplate
base theme = bartik
version = 0.2
stylesheets[all][] = css/coolest.css
stylesheets[all][] = css/colors.css
stylesheets-conditional[IE][all][] = ie9-and-below.css

So here it is added as the last CSS file and the Internet Explorer target is any version and the file is named and stored in the same directory as the .info file.

To target various Internet Explorer versions you would use similar to:

stylesheets-conditional [IE][all][] = cssname.css IE All
stylesheets-conditional [lt IE 7][all][] = cssname.css  IE 6 & below
stylesheets-conditional [IE 9][all][] = cssname.css IE9
stylesheets-conditional [gt 7][all][] = cssname.css IE 8 & above  

To take effect flush the cache.

Drupal 6

For Drupal 6 and IE css styles download and install the Drupal version 6 of conditional styles.

Once installed, open your theme .info style and add in stylesheet declarations like so:

conditional-stylesheets[if lt IE 7][all][] = ie6-and-below.css
conditional-stylesheets[if IE 8][all][] = ie8.css
conditional-stylesheets [if gt IE 5.5[all][] = ie6-and-above.css

The naming is different from that of Drupal 7 - for all instances - save the .info - flush the Drupal cache and obviously (or not) create the actual CSS file with style declarations.