E[foo]        an E element with a "foo" attribute (CSS 2)
E[foo="bar"]  an E element whose "foo" attribute value is exactly equal to "bar" (CSS 2)
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" (CSS 2)
E[foo^="bar"] an E element whose "foo" attribute value begins exactly with the string "bar" (CSS 3)
E[foo$="bar"] an E element whose "foo" attribute value ends exactly with the string "bar" (CSS 3)
E[foo*="bar"] an E element whose "foo" attribute value contains the substring "bar" (CSS 3)
E[foo|="en"]  an E element whose "foo" attribute has a hyphen-separated list of values beginning (from the left) with "en" (CSS 2)