must have either a .htm or a .html file extension. It doesn't really matter which extension you choose - the .htm extension is really a leftover from the days when software would only allow files to be saved with a three-letter file extension.
The first tag in any HTML document is <html>. This tells a browser that it is the start of an HTML document. The last tag in the HTML document is </html> and tells your browser that it has reached the end of the HTML document.