KHTML is the HTML-rendering engine developed by the KDE development team.
Based on the Qt toolkit.
Open source, under LGPL.
Apple used it as a basis for their Safari browser, which is the default
browser for Mac OS X.
Market share: a small percent of the UNIX workstations, and a relatively
large percent of people using Mac OS X.
Made a policy of trying to support MSIE extensions to the standard (so
MSIE-only sites will be compatible), as long as they don't contradict the
W3C standards. (still many sites break.)
Historically, many bugs were present, but the situation will hopefully
improve in the future.
Verdict
If your client workstation is Windows, you may have problems testing it. (
KDE may or may not be ported to cygwin and a Win32-based X-Windows server).
The problem is that many pages can break in it because of its many bugs.
I recommend testing in it, but it's not absolutely necessary.