Respond.js provides IE 6-8 support for
max-width CSS3 media queries.
Respond.js only works on
<link> tags; it will not work on
@import stylesheet calls. Drupal 7 uses
@import when CSS aggregation is disabled, but uses
<link> when CSS aggregation is enabled.
So go to /admin/config/development/performance, and enable Aggregate and compress CSS files.