Titles are important because they are displayed in search engine results as well as in the browser's toolbar.
This rule verifies that the <head> tag contains a <title> one, and the <html> tag a <head> one.
The following code snippet:
<html> <!-- Non-Compliant --> <body> ... </body> </html>
should be refactored into:
<html> <!-- Compliant --> <head> <title>Some relevant title</title> </head> <body> ... </body> </html>