HTML doctype declaration
All HTML documents must start with a <!DOCTYPE> declaration. The declaration is not an HTML tag. It is an "information" to the browser about what document type to expect.
Document type declaration - Wikipedia
A document type declaration, or DOCTYPE, is an instruction that associates a particular XML or SGML document (for example, a webpage) with a document type definition (DTD) (for example, the formal definition of a particular version of HTML 2.0 - 4.0).
Doctypes and markup styles - W3C Wiki
The doctype appears just above the <html> tag, at the very start of each document you write: <!DOCTYPE html> <html lang="en-GB"> <head> <meta charset="utf-8"> <title>My fabulous document</title> ... etc.
"<!DOCTYPE html>" What does it mean? - Stack Overflow
<!DOCTYPE html> is the doctype for the document. It basically tells the web browser how to render DOCTYPE html> does not conform to the requirements of that. This is unlike <!DOCTYPE HTML...
HTML: <!DOCTYPE> tag
The HTML <!DOCTYPE> tag is the very first line in the HTML document. It tells the browser what version of HTML the document is written so that the browser knows what to expect.
Doctype - Словарь | MDN
Doctype. Select your preferred language. Русский English (US). Смотрите также. Основная информация. Definition of the DOCTYPE in the HTML specification.
DOCTYPE HTML 4.01 tutorial - HTML tutorials - w3resource
Document Type Declaration or DOCTYPE declares which version of HTML is being followed by the document in question. Each version of HTML has its own set of rules, exclusions and inclusions.