width: 100%;
<!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> <meta http-equiv='Content-Type' content='text/html; charset=UTF-8' /> <title>Experimenting with Percentage Measurement</title> <style type='text/css'> div { width: 100%; background: black; color: white; } </style> </head> <body> <div>What happens when I apply a 100% width?</div> </body> </html>