It's best to disable all plugins before doing these tests.

The tests in this directory are pretty ad-hoc. The tests in the css/
subdirectory are specific to linking in stylesheets. The tests in the
sniffing/ subdirectory were written to test how UAs handled edge cases
such as mis-labelled binary files.

NOTE: The tests in this directory generally assume that there should
be no sniffing at all going on. However, HTML5 introduces a limited
set of sniffing behaviours to the rules. These tests will need to be
updated to match that spec.