doctype-first
If set, the first element in the file must be <!DOCTYPE ... > (excluding comments and whitespace).
The following patterns are considered violations:
<html>
  <head>
    ...
  </head>
  <body>
    ...
  <boby>
</html>
<html>
  <head>
    ...
  </head>
  <body>
    ...
  <boby>
<html>
<!DOCTYPE html>
The following patterns are not considerd violations:
<!DOCTYPE html>
<html>
  <head>
    ...
  </head>
  <body>
    ...
  <boby>
</html>
Options
You can pass the "smart" as value for the rule. This value allow the none presence of a <!DOCTYPE ... > but only if the tag <head> is not present.
Given :
  "doctype-first": ["error", "smart"]
The following patterns are considered violations:
<html>
  <head>
    ...
  </head>
  <body>
    ...
  <boby>
</html>
<html>
  <head>
    ...
  </head>
  <body>
    ...
  <boby>
<html>
<!DOCTYPE html>
The following patterns are not considerd violations:
<section>
  ...
</section>