header.html

HTML page Reference

An .html file contains a web page in HTML, the standard markup language of the web. The file is plain text with elements in angle brackets. A web browser parses the file and shows text, images, and links. Styles and scripts attach through CSS and JavaScript. The WHATWG maintains HTML as a living standard.

A file extension is only a part of the file name. It does not give a guarantee about the format or the content of a file. But on Windows, the extension selects the program that opens the file.