This may be very, very basic, but I've never seen a great explanation of when and what to put in the <head> of an html email in terms of the doctype, html attribute, and meta tags.

For example, the html email boilerplate uses:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0"/>

I personally use a stripped-down approach with no doctype or meta tags and a simple <html> tag. This is possibly not a good practice, however I also haven't seem to have encountered any issues because of it, either with Litmus or in actual sends.

Thanks so much for any insight here.