Cascading Style sheet or CSS is one of the most used and convenient way of coding the mark up languages. Most of the websites are built on an HTML platform and hence it is important for us that the CSS is also correctly applied so that the information that the website wants to transmit to its viewers is correct. But, designers often commit fundamental mistakes that can make things go terribly wrong for a website. The css-faq.com mentions some of the common mistakes occurred in CSS known as CSS Mistakes.
You don’t waste bytes by adding units such as px, pt, em, etc, when the value is zero. The only reason to do so is when you want to change the value quickly later on, otherwise declaring the unit is meaningless. Zero pixels is the same as zero points. In order to get the accurate choice of the color, you must specify the hex code rather than writing the color in plain English. Never forget to add hash # at the start of the code so that it can be parsed correctly. Otherwise, you will have to remember to add another hash to prevent errors.
Unless you are likely to be changing your code much, avoid using several lines when only one line can serve your purpose....