.htm or .html extension - which one is correct and what is different?

Question

When I save a file with an .htm or .html extension, which one is correct and what is different?

Answer 1

When you save the file locally, the difference doesn't matter - your local system will likely treat the two file extensions as interchangeable for loading by your browser. The reason for it is that historically Windows-based systems used 3 letter extensions (htm) and Unix-based systems the 4 letters (html).

On a server-side, there may be some differences when it comes to serving default filenames:

The one situation in which there may be a difference between the two extensions is that of a server's default filenames. When a URL that does not specify a filename is requested from a server, such as http://www.domain.dom/dirname/, the server returns a file from the requested URL that matches a default filename. Examples of common default filenames include "index.html," "index.htm," "default.html," "default.htm," etc. However, an administrator can make the server's default filename anything he/she so desires.

Note that servers are often configured with more then one default filename.

So if you have any level of control over your server's default filenames, then this shouldn't be an issue.