Square brackets in CSS [duplicate]

Question

What does it mean when something between square brackets in CSS? E.g.

input[type="radio"]

Answer 1

It's an attribute selector in CSS

E[foo="warning"] Matches any E element whose "foo" attribute value is exactly equal to "warning".

more on http://www.w3.org/TR/CSS2/selector.html

Answer 2

Square brackets are attribute selector syntax.

Your (complete) example means "Select elements of type input which have a type attribute with the value radio" e.g. <input type="radio">