HTM or HTML Extension?

[beingchinmay]

When you save an HTML file, you can use either the .htm or the .html extension. The .htm extension comes from the past when some of the commonly used software only allowed three letter extensions. It is perfectly safe to use either .html or .htm, but be consistent. mypage.htm and mypage.html are treated as different files by the browser.

[Edu India]

HTM is used only as an alternate extension to .HTML. This happens for some reasons, for examples, in some operating systems, like the Disk Operating System and Window 3.X; they do not allow the use of four-letter extensions.
HTML for some operating systems and servers that do not accept four-letter extensions.
Although today, operating systems have developed, and can now support long file names and four-letter file extensions.
HTM and HTML refer to file extensions of HTML files. They are files of a plain text type.