An id is used but once per document.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> <html xmlns='http://www.w3.org/1999/xhtml' xml:lang='en'> <head> <title>Class and ID Selectors</title> <style type='text/css'> body { font-family: sans-serif; } p.container { border: 1px solid rgb(29, 179, 82); background: rgb(202, 222, 245); padding: 10px; width: 245px; height: 245px; float: left; margin: 10px; } p#container-1234 { border: 1px solid rgb(154, 232, 181); background: rgb(82, 157, 245); } </style> </head> <body> <p class='container'> A class represents something that you can have more than one of. </p> <p class='container' id='container-1234'> An id is used but once per document. </p> </body> </html>