There's a little known bug in IE 6,7,8,9 that means if you have more than 4095 selectors in a single CSS file then the following style rules will be simply ignored.

Moodle's styles.php puts all the styles from the theme and modules into a single file, following the best practice of the web to reduce download times and increase rendering times.

But it doesn't do anything if Themes go over this limit. Simply splitting the CSS into two files is enough to work around this bug.

A test file that shows the bug:
http://marc.baffl.co.uk/browser_bugs/css-selector-limit/index.html

A tool to check how many selectors in a CSS file via upload or url:

http://snippet.bevey.com/css/selectorCount.php

And some selector counts from standard themes:

Standard: 4649
Arialist: 4219
Moodle.org: 6031

So it looks like nearly every Moodle theme has sections that are simply ignored on IE < 10, and Moodle.org has nearly 2000 selectors, almost a third of the total, that are being ignored.

If you were to use just the "Base" theme on it's own (not possible by default) then it, in conjunction with the module CSS would use 3720 selectors, leaving room for 376 selectors for any theme descending from Base, assuming is not excluding any parent theme sheets, before you hit this limit.