CSS3/Selectors/attribute selector/attribute=value
[attributename="val"]
This selector represents an element with the "attributename" attribute whose value is exactly "val".
Syntax
elementname[attributename="val"]{ properties }
Example
In the following example, the selector represents a span element whose class attribute has exactly the value "example":
[style.css]
span[class="example"]{ color: red; font-size: 1.2em; }
[index.html]
<body> <p>This is a <span class="example">paragraph</span>.</p> <p>This is a <span class="test">paragraph</span>.</p> </body>
The CSS Selectors Level 3 specification defines the attribute selector in 6.3.1. Attribute presence and value selectors.