.html and .htm both denote files containing HyperText Markup Language, the core code for structuring web pages. They represent the same HTML file format and content. The difference is purely in the file extension's length. Historically, older disk operating systems like DOS limited file extensions to three characters, leading to the adoption of ".htm". Modern operating systems support longer extensions, making ".html" the common standard today. All web browsers treat both extensions the same, recognizing the content as HTML and rendering the web page identically.

In practice, you may encounter both extensions depending on the origin or platform. Legacy Windows environments, particularly older website authoring tools like Microsoft FrontPage, often defaulted to using ".htm" files. However, virtually all modern web development tools, text editors (like Visual Studio Code or Sublime Text), content management systems (like WordPress), and web servers use ".html" by default. When you save a new HTML file in your editor, it will likely use ".html".

The key advantage of ".html" is its clarity as the standard abbreviation for "Hypertext Markup Language", reducing potential confusion. ".htm" persists primarily for backward compatibility with very old systems, but this is rarely a limitation today. There are no inherent performance, security, or ethical differences. Modern web infrastructure treats them interchangeably. ".html" is the overwhelming choice for new files and future development, solidifying its position as the conventional extension for HTML documents.