File with a .HTML contains source code of a web site written with the Hypertext Markup Language – text, references to graphics, charts and formatted elements, displayed by a web browser on a static web site. The code is written in text format with tags which dictate the structure and formatting of a web site.

HTML web sites can be opened within the notepad, however, for this purpose usually dedicated software is used. Such software can make the work easier, they give hints and highlight fragments of syntax and they contain templates. The index.html files are utilised as the start pages for the web browsers demanding access to catalogues of a given web site. In case of Windows WWW servers, these are the default.html files. HTML code is analysed by a web browser and displayed in a proper form to a viewer, and if a viewer wants to see its text form, specific option has to be chosen in the browser.