Zombie.js uses Sizzle.js which provides support for most CSS 3 selectors with a few useful extension.
Sizzle.js is the selector engine used in jQuery, so if you're familiar with jQuery selectors, you're familiar with Sizzle.js.
The following list summarizes which selectors are currently supported:
* Any element
E An element of type E
E#myid An E element with ID equal to "myid"
E.foo An E element whose class is "foo"
E[foo] An E element with a "foo" attribute
E[foo="bar"] An E element whose "foo" attribute value is exactly equal to "bar"
E[foo!="bar"] An E element whose "foo" attribute value does not equal to "bar"
E[foo~="bar"] An E element whose "foo" attribute value is a list of whitespace-separated values, one of which is exactly equal to "bar"
E[foo^="bar"] An E element whose "foo" attribute value begins exactly with the string "bar"
E[foo$="bar"] An E element whose "foo" attribute value ends exactly with the string "bar"
E[foo*="bar"] An E element whose "foo" attribute value contains the substring "bar"
E[foo|="en"] An E element whose "foo" attribute has a hyphen-separated list of values beginning (from the left) with "en"
E:nth-child(n) An E element, the n-th child of its parent
E:first-child An E element, first child of its parent
E:last-child An E element, last child of its parent
E:only-child An E element, only child of its parent
E:empty An E element that has no children (including text nodes)
E:link A link
E:focus An E element during certain user actions
E:enabled A user interface element E which is enabled
E:disabled A user interface element E which is disabled
E:checked A user interface element E which is checked (for instance a radio-button or checkbox)
E:input An E element that is an input element (includes textarea, select and button)
E:text An E element that is an input text field or text area
E:checkbox An E element that is an input checkbox
E:file An E element that is an input file
E:password An E element that is an input password
E:submit An E element that is an input or button of type submit
E:image An E element that is an input of type image
E:button An E element that is an input or button of type button
E:reset An E element that is an input or button of type reset
E:header An header element, one of h1, h2, h3, h4, h5, h6
E:parent A parent element, an element that contains another element
E:not(s) An E element that does not match the selector s (multiple selectors supported)
E:contains(t) An E element whose textual contents contains t (case sensitive)
E:first An E element whose position on the page is first in document order
E:last An E element whose position on the page is last in document order
E:even An E element whose position on the page is even numbered (counting starts at 0)
E:odd An E element whose position on the page is odd numbered (counting starts at 0)
E:eq(n)/:nth(n) An E element whose Nth element on the page (e.g :eq(5))
E:lt(n) An E element whose position on the page is less than n
E:gt(n) An E element whose position on the page is less than n
E F An F element descendant of an E element
E > F An F element child of an E element
E + F An F element immediately preceded by an E element
E ~ F An F element preceded by an E element