How to detect .Net Framework installed in a web client?

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In my last post I talked about how to detect .Net framework installed in your local machine. This is another .Net framework detection. But not for you, it is for web servers. Suppose you browse some web site. How does that web site know if you have installed .Net framework? If you did, which version of .Net framework have you installed?

Whenever .Net framework is installed, it adds a string to Internet Explorer's user agent. For example, I have both 1.0 and 1.1 .Net framework installed. The user agent of my IE browser will be “User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1;   .NET CLR 1.0.3705;   .NET CLR 1.1.4322)” .

The user agent string is added at this reg key [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\5.0\User Agent\Post Platform]

Of course, this works only for IE.