Use the scheme attribute to define formats for the content attribute:

Edit code and click me	Your Result (Some examples do not work in Google Chrome):
<html> <head> <meta name="date" content="2010-01-02" scheme="YYYY-MM-DD" /> <meta name="identifier" content="0-2345-6634-6" scheme="ISBN" /> </head> <body> </body> </html>
Edit the code above and click to see the result	www.xhtml.co.il - Try it yourself