Cascading StyleSheets allow you to dress up / style your HTML.
The 'cleaner' your HTML is written, the easier it will proove to apply CSS styling.
Just like with HTML, the vocabular and grammar to this 'language' are restricted.
To put it better; it's easy to separate easy, hard to very hard CSS subject matter
into our different Educational Services levels, Introduction to, Advanced and Expert.
Yet, after completing every level course, you'll be able to write CSS.