According to recommendations from the W3C, for the creation of a website, to be the skeleton of the web you need to use HTML and for the design of your website is required to use CSS.
For example :
If you are creating a web article to align the center, then in writing on your HTML is not advisable to make the tag 'Align' but just enough time to write his article and to make so that the article according to your design using CSS.
The conclusion is:HTML to build the basic framework and writing articles only. CSS is used to design web pages. Next, you can learn the Rules of Writing CSS.