<!doctype>

The 1st element of the HTML code in an HTML email. It's more a browser-specific statement than a tag. It gives to the browser an indication about the HTML version in which code is written.

There are several possible declarations.

HTML 5 : <!DOCTYPE HTML>
HTML 4.01 Strict : <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
HTML 4.01 Transitional : <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
HTML 4.01 Frameset : <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "http://www.w3.org/TR/html4/frameset.dtd">
XHTML 1.0 Strict : <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
XHTML 1.0 Transitional : <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
XHTML 1.0 Frameset : <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd">
XHTML 1.1 : <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">

Without Doctype, it's "Quirks Mode". With a Doctype, we switch to "Standards Mode". According to it, differences in code interpretation may appear on some webmail platforms. For example, with "Quirks Mode", CSS class names and identifiers are not case-sensitive. So, following three selectors are equivalent in "Quirks Mode", but not in "Standards Mode".

.foo {background:blue;}
.Foo {background:white;}
.foO {background:red;}

We must distinguish several types of mail clients:

Tips

We recommend using the HTML5 doctype. It trigger "Standards Mode" in the relevant mail clients.

<!DOCTYPE HTML>

A question?