How to combine class and ID in CSS selector?

In your stylesheet:

div#content.myClass

Edit: These might help, too:

div#content.myClass.aSecondClass.aThirdClass /* Won't work in IE6, but valid */
div.firstClass.secondClass /* ditto */

and, per your example:

div#content.sectionA

Edit, 4 years later: Since this is super old and people keep finding it: don't use the tagNames in your selectors. #content.myClass is faster than div#content.myClass because the tagName adds a filtering step that you don't need. Use tagNames in selectors only where you must!

There's nothing wrong with combining an id and a class on one element, but you shouldn't need to identify it by both for one rule. If you really want to you can do:

#content.sectionA{some rules}

You don't need the div in front of the ID as others have suggested.

In general, CSS rules specific to that element should be set with the ID, and those are going to carry a greater weight than those of just the class. Rules specified by the class would be properties that apply to multiple items that you don't want to change in multiple places anytime you need to adjust.

That boils down to this:

 .sectionA{some general rules here}
 #content{specific rules, and overrides for things in .sectionA}

Make sense?

Well generally you shouldn't need to classify an element specified by id, because id is always unique, but if you really need to, the following should work:

div#content.sectionA {
    /* ... */
}

Ids are supposed to be unique document wide, so you shouldn't have to select based on both. You can assign an element multiple classes though with class="class1 class2"

You can combine ID and Class in CSS, but IDs are intended to be unique, so adding a class to a CSS selector would over-qualify it.

Continuing Ajm's answer:

For a dynamic id or class selection combination ->

div[id*='**myStandarKey-**'].myClass{

}

/* This is to validate something like: */

div[id*='myStandarKey-**12**'].myClass{

}

/* AND */

div[id*='myStandarKey-**13**'].myClass{

}

/* etc. */

.sectionA[id='content'] { color : red; }

Won't work when the doctype is html 4.01 though...