Long number for if you allow a bad guy to run active content on your website it's not your website anymore.
At present this rule seems a bit archaic.
Twelve years ago a Web site defacement was a large scale problem discussed by the press and mentioned
on TV.
You can see one example of web site defacement in the slide.
This proves that the rule was at some point valid.
Nowadays there's nothing special in placing your own content on a Web site.
We're living in the second era of the Internet.
Most web sites are interactive administrators and developers learn that you can limit attempts to gain
control over web servers by allowing Husna content on the Web site.
Right now you don't need control over the web server to modify the site's content as was in the past.
The risk still comes from users visiting a given site.
This is the basis of one of the most frequent attacks ever cross-site scripting.
It consists of injecting client side scripts into trusted Web sites or in changing links to these sites.
The attack is conducted through the site itself or through a link to the site despite mentioned reservations.
The fourth law is still valid if someone can without restriction modify the content of a Web site he
or she creates a threat for the web server and for the users of the Web site.
It's not the same however as gaining control over the web server.