Expect everything to change...

1. X Y

li a {
  text-decoration: none;
}

Target the anchors which are within an unordered list? This is specifically when you’d use a descendant selector.

2. X:visited and X:link

a:link { color: red; }
a:visted { color: purple; }

We use the :link pseudo-class to target all anchors tags which have yet to be clicked on.

3. X + Y

ul + p {
   color: red;
}

This is referred to as an adjacent selector. It will select only the element that is immediately preceeded by the former element. In this case, only the first paragraph after each ul will have red text.

4. X > Y    (direct children)

div#container > ul {
  border: 1px solid black;
}

A selector of #container > ul will only target the uls which are direct children of the div with an id of container. It will not target, for instance, the ul that is a child of the first li.
For this reason, there are performance benefits in using the child combinator. In fact, it’s recommended particularly when working with JavaScript-based CSS selector engines.

5. X[title]

a[title] {
   color: green;
}

Referred to as an attributes selector, in our example above, this will only select the anchor tags that have a title attribute.

6. X[href="foo"]

a[href="http://net.tutsplus.com"] {
  color: #1f6053; /* nettuts green */
}

The snippet above will style all anchor tags which link to http://net.tutsplus.com; they’ll receive a branded green color. All other anchor tags will remain unaffected.

7. X[href*="nettuts"]

a[href*="tuts"] {
  color: #1f6053; /* nettuts green */
}

There we go; that’s what we need. The star designates that the proceeding value must appear somewhere in the attribute’s value. That way, this covers nettuts.com, net.tutsplus.com, and even tutsplus.com.

8. X[href^="http"]

a[href^="http"] {
   background: url(path/to/external/icon.png) no-repeat;
   padding-left: 10px;
}

If we want to target all anchor tags that have a href which begins with http, we could use a selector similar to the snippet shown above. This is a cinch with the carat symbol. It’s most commonly used in regular expressions to designate the beginning of a string.

9. X[href$=".jpg"]

a[href$=".jpg"] {
   color: red;
}

Again, we use a regular expressions symbol, $, to refer to the end of a string. In this case, we’re searching for all anchors which link to an image — or at least a url that ends with .jpg. Keep in mind that this certainly won’t work for gifs and pngs.

10. X:checked

input[type=radio]:checked {
   border: 1px solid black;
}

This pseudo class will only target a user interface element that has been checked - like a radio button, or checkbox. It's as simple as that.

11. X:after

The before and after pseudo elements kick butt. Every day, it seems, people are finding new and creative ways to use them effectively. They simply generate content around the selected element.
Many were first introduced to these classes when they encountered the clear-fix hack.

.clearfix:after {
    content: "";
    display: block;
    clear: both;
    visibility: hidden;
    font-size: 0;
    height: 0;
 }

.clearfix {
   *display: inline-block;
   _height: 1%;
}

This hack uses the :after pseudo element to append a space after the element, and then clear it. It's an excellent trick to have in your tool bag, particularly in the cases when the overflow: hidden; method isn't possible.

12. X:hover

div:hover {
  background: #e3e3e3;
}

Oh come on. You know this one. The official term for this is user action pseudo class. It sounds confusing, but it really isn't. Want to apply specific styling when a user hovers over an element? This will get the job done!

13. X:not(selector)

div:not(#container) {
   color: blue;
}

The negation pseudo class is particularly helpful. Let's say I want to select all divs, except for the one which has an id of container. The snippet above will handle that task perfectly.

14. X::pseudoElement

p::first-line {
   font-weight: bold;
   font-size: 1.2em;
}

We can use pseudo elements (designated by ::) to style fragments of an element, such as the first line, or the first letter. Keep in mind that these must be applied to block level elements in order to take effect.

15. X:nth-child(n)

li:nth-child(3) {
   color: red;
}

Remember the days when we had no way to target specific elements in a stack? The nth-child pseudo class solves that!
Please note that nth-child accepts an integer as a parameter, however, this is not zero-based. If you wish to target the second list item, use li:nth-child(2).
We can even use this to select a variable set of children. For example, we could do li:nth-child(4n) to select every fourth list item

16. X:first-child

ul li:first-child {
   border-top: none;
}

This structural pseudo class allows us to target only the first child of the element's parent. You'll often use this to remove borders from the first and last list items.
For example, let's say you have a list of rows, and each one has a border-top and a border-bottom. Well, with that arrangement, the first and last item in that set will look a bit odd.
Many designers apply classes of first and last to compensate for this. Instead, you can use these pseudo classes.

17. X:first-of-type

The first-of-type pseudo class allows you to select the first siblings of its type.

A Test

To better understand this, let's have a test. Copy the following mark-up into your code editor:

<div>
   <p> My paragraph here. </p>
   <ul>
      <li> List Item 1 </li>
      <li> List Item 2 </li>
   </ul>

   <ul>
      <li> List Item 3 </li>
      <li> List Item 4 </li>
   </ul>
</div>

Now, without reading further, try to figure out how to target only "List Item 2". When you've figured it out (or given up), read on.

Solution 1

There are a variety of ways to solve this test. We'll review a handful of them. Let's begin by using first-of-type.

ul:first-of-type > li:nth-child(2) {
   font-weight: bold;
}

This snippet essentially says, "find the first unordered list on the page, then find only the immediate children, which are list items. Next, filter that down to only the second list item in that set.

Solution 2

Another option is to use the adjacent selector.

p + ul li:last-child {
   font-weight: bold;
}

In this scenario, we find the ul that immediately proceeds the p tag, and then find the very last child of the element.

Solution 3

We can be as obnoxious or as playful as we want with these selectors.

ul:first-of-type li:nth-last-child(1) {
   font-weight: bold;
}

This time, we grab the first ul on the page, and then find the very first list item, but starting from the bottom!


 my thanks to:
http://net.tutsplus.com/tutorials/html-css-techniques/the-30-css-selectors-you-must-memorize/?utm_source=feedburner&utm_medium=email&utm_campaign=Feed%3A+nettuts+%28Nettuts%2B%29