To reduce the number of HTTP calls and increase the performance of
a website, developers can compile all of a sites CSS into a single
file and serve it once. This can result in a large number of rules and
selectors in a single file.
Before taking this performance optimisation approach, you should be aware
that Internet Explorer has limits on the amount of CSS that it will process
and will ignore any extra rules in order to limit memory usage as it's TL;DR
(too long, didn't read).