CSS Attribute Selectors: Built-In Classes
http://www.cssnewbie.com/category/css-rules/

Glazebrook, Rob L.
CSSnewbie
2008

Abstract:

By using attribute selectors in your CSS, you’re able to target elements with specific attributes, or even specific values within those attributes. When using attribute selectors, the attribute is contained within [brackets], just like how .classes have a leading period, or #ids have a leading pound sign.