As a beginner, it is very common that you will make mistake in writing your CSS code. Incorrect or non-standard code may cause unexpected results in how your page displayed or functions in a web browser.
The World Wide Web Consortium (W3C) has created a great tool http://jigsaw.w3.org/css-validator/
to automatically check your style sheets, and point out any problems/errors your code might have, such as invalid CSS property missing closing bracket or missing semicolon (;) etc. It is absolutely free.
Why Validate Your HTML Code?
There are a number of reasons why you should validate your code.
1. It Helps Cross-Browser, Cross-Platform, and Future Compatibility.
2. A good quality website increases search engine visibility.
3. Professionalism: As a web developer, your code should not raise errors while seen by the visitors.