What is the difference between the extension. Html and. Htm?

From a practical point of view there is no difference. Both extensions (. Html and. Htm) formatttato identify a document to HTML.

The reason for the existence of two equivalent extensions must be sought in the past: the old 8-bit operating systems (like Windows 3.1) only accept files with extension up to 3 characters. For this reason the extension was born. Htm (a. Html would not be recognized as valid).

To date, the two extensions are completely equivalent, and both are recognized by modern OS as an HTML file.