W3Schools is a mixed bag. Some good information, some bad practices. The CSS info is probably good, but you should read some other sources for comparison. W3 has a page dedicated to
resources for learning CSS, but I don't know if they're any better than W3Schools.
Once you're familiar with CSS, you'll want to keep a link to the
CSS specs close at hand, since they're the final word on what CSS is and does.