What's the difference between htm file and html file?

Answer 1

.htm and .html are the extensions of HTML files. The only difference between the two is that .htm is used as an alternate to .html by few servers that do not accept four character extensions.
operating systems like Disk Operating System and Window 3.X, they do not allow the use of four-letter extensions.
https://www.quora.com/What-is-the-difference-between-a-html-and-a-htm-page
http://www.differencebetween.net/technology/internet/difference-between-htm-and-html/

Answer 2

there is no difference it is just like "src"---"source" if you want difference then there is a difference "htm" has "L" missing and "htmL" has "L"