Best way to select items with CSS

I have read all about the best order of selectors in CSS but I often (all the time) see people doing this....

nav ul li#email-me a {
   width: 120px;
   height: 43px;
   background: url(images/email_me.png) no-repeat top left;
   display: block;
}

From what I have read, my understanding is that this is better for performance...

#email-me a {
   width: 120px;
   height: 43px;
   background: url(images/email_me.png) no-repeat top left;
   display: block;
}

Am I missing something? Is the second method better? If it is, why does everybody use the first method?

Answers

I often use the first method, because

• As mentioned in some other answers, it's more specific. An ID doesn't automatically make an entire rule more specific than any others forever; you can add a type selector — or pretty much anything except the universal selector * — to the ID and it'll immediately stomp out a rule with a lone ID selector. I don't find myself increasing a selector's specificity on purpose with this technique as much as I do it for organization, though...

• To elaborate on organization: this kind of rule is often related to other rules that start with nav ul, and it helps to visually group them with a few initial selectors that are already in place.

  For example:

  nav ul {
      /* Styles for the ul */
  }
  
  nav ul li {
      /* Styles for the items */
  }
  
  nav ul li a {
      /* Styles for the item links */
  }
  
  /* 
   * If the selector here were just #email-me a, it'd be impossible
   * to see, right away, how it's related to the above rules, without
   * studying the markup that it applies to, and so on.
   */
  nav ul li#email-me a {
      /* Styles for the link in the #email-me item */
  }
  

The second method is supposedly faster because no additional checks need to be performed after identifying the ancestor element with the ID, but I have no benchmarks and I can't be bothered to make any.

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