E-Mail is not the Web. Usenet is not the Web.
If you see this page, it's probably because you're using a program that thinks interpreting plain-text protocols such as e-mail and Usenet as if they were pages on the WWW is a good idea. Not so.
Most importantly, it's not what those protocols are designed for. HTML-encoded e-mail or Usenet posts are near-impossible to read without acquiring and installing a specific program. Currently all of these programs suck. Even if you happen to like a program that likes HTML, it's rather rude to demand that I use such a program.
Equally important to the main issue is the topic of security. HTML itself is reasonably safe, but most programs that think it's applicable outside the Web also think it's good to add things like "active content". Usually these extensions are chock full of security holes. Click here to see if you're vulnerable to the most silly denial-of-service attack. The combination of browser security hole and use of HTML outside of it's intended arena means that anyone who uses an HTML-ish mail/news program is now exposed to attack while reading e-mail. Remember the Good Times e-mail virus hoax? Now it's real.