You've missed something.

First, you didn't say where you "downloaded a website" from, other than
"Microsoft," so we have no idea where you got what you're posting, and
therefore, have no way of finding out what the reference to a CssClass might
have come from or referred to.

Here's a tip: Pretend we are all total strangers (a safe bet) and that we
know nothing about your situation (another safe bet). Tell us anything that
might contribute to finding out the nature of the problem. In this case, you
say that "there are no css files in the project." What project? Where did
you get it from? Saying "Microsoft" is like saying you got a book from the
Library of Congress. Be as specific and detailed as possible.

Solving any problem or puzzle is basically the same process that is used to
solve mathematical problems. Without certain data, the problem cannot be
solved. Example:

X + Y = 100.
Y is a number between -50.333 and 50.666.
What is the value of X?

Css styles can also be placed in a style tag in the page, so there
doesn't need to be any css file.

If there are no style tag in the page, look for a link tag that
references a css file. If there is no link tag either, the CssClass
property is probably just something that was accidentally left in the code.