In my email view I usually just do something like this
<dl> <dt>Name</dt> <dd>Value</dd> </dl>
Should I do this?
<html> <head></head> <body> <dl> <dt>Name</dt> <dd>Value</dd> </dl> </body> </html>
In other words, it's like I'm marking up a separate document?
I think I can safely assume that any web-based email client will remove it?
What is the correct approach?
The correct way is to follow the HTML standard. You can verify your HTML page here.
Your mail client should follow it and should discard unsupported or unsafe content, such as JavaScript.
I will reveal some of the reasons why the following standards may be beneficial: