Is this:

ul#mainNav {
margin: 0;
padding: 0;
list-style: none;
}

the same as this:

#mainNav ul {
margin: 0;
padding: 0;
list-style: none;
}

If so, why use one instead of the other?

No they are not the same.

the first would apply to a ul element with an id of mainNav, and the second would apply to a ul element that is a child element of a parent with an id of mainNav.

Originally Posted by chrishirst

No they are not the same.

the first would apply to a ul element with an id of mainNav, and the second would apply to a ul element that is a child element of a parent with an id of mainNav.

Wouldn't they have the same effect though?

the rules are identical so, yes the effect would be the same on the elements the CSS rules apply to.