The browser is an expensive option that offers little layouting control to display emails, therefore we should avoid using it if we can.
The plan is:

• Always convert plaintext to richtext (so we can highlight links and stuff)
• Only use a webview if the html is sufficiently complex
• Use a TextEdit with richtext in all other cases.

Open problems:

• Heuristics to detect when the TextEdit is no longer capable of displaying the mail
• gmail uses custom html quoting and I'm not sure we can deal with that in a TextEdit