The thing is, people want to read html emails. MS outlook doesn't need a web browser to run, because it uses ... MS Word to render emails, which if you've ever tried to send formatted emails programmatically you'll know it's a dark, dark state of affairs.
Good. Pretty much all emails I care about would be better off without any formatting beyond what e.g. Markdown provide - and that subset of HTML works well enough in Outlook.
I dread the day that Google manages to grab enough of Outlook's marketshare so that they can dictate the direction of email just like they have been doing to the web - we're almost there already.
