An HTML document is a text file with a name ending in .html, for example, hello.html. A modern, minimal HTML document looks like this:
<!DOCTYPE html> <html lang="en"> <head> <meta charset="utf-8"/> <title>Hello, world</title> </head> <body> <h1>Hello, World</h1> </body> <html>
The first line specifies the document type (<!DOCTYPE html>). Today, this can be as simple as the word html telling a browser that this file is to be interpreted as an HTML5 document. Documents written to the older specifications contain the name of that spec followed by a path to a Document Type Definition (DTD) file that can be used to examine the document for conformance. Things are a lot more flexible these days.