html - Why does an anchor tag's href values need http:// preprended to the URL?

Question

<a href="www.stackoverflow.com" target="_blank">click here</a>

Clicking the above link on a site's HTML page would try to take the user to

<site>/index.html/www.stackoverflow.com

Where as following works fine

<a href="http://www.stackoverflow.com" target="_blank">click here</a>

What is the rationale for this behavior?

Answer 1

There are several protocols: HTTP, HTTPS, FILE, SSH, SSL, FTP. In addition, as Jeremy mentioned, it's quite possible you may have a file on your server with the exact name of the text you're entering in the HREF.

Basically, any text in the href without a protocol is assumed to be a relative path if there is no / or protocol.