For IE users, the CSS links always have svg=0. So if you have a theme with SVG content this is not served (you get a 404 response). IE 9 or above can display SVG so presumably they should get this content?

An example link in a Moodle site home page:
<link rel="stylesheet" type="text/css" href="http://learning.example.com/theme/styles.php?theme=my_theme&rev=1394802516&type=theme&svg=0" />

The equivalent link served for Firefox:
<link rel="stylesheet" type="text/css" href="http://learning.example.com/theme/styles.php/my_theme/1394802516/all" />