[attributename]

This selector represents an element with a "attributename" attribute, whatever the value of the attribute.

Syntax

elementname[attributename]{ properties }

The following attribute selector represents an p element that carries the title attribute, whatever its value:

[style.css]

 p[title]{
   color: red;
   font-size: 1.2em;
 }

[index.html]

<body>
  <p title="sample text">This is a paragraph.(with title attribute)</p>
  <p>This is a paragraph.</p>
</body>

The CSS Selectors Level 3 specification defines the attribute selector in 6.3.1. Attribute presence and value selectors.