During the process of learning web design, I did not go into many problems I encountered, such as the relationship between html and html. I specially sorted out today, hoping that it will be helpful to you.
The most commonly used extension for files containing HTML content is .html, but older operating systems like DOS limit the extension to a maximum of 3 characters, so the .htm extension is also used. Although it is used less now, the .htm extension is still generally supported.
Both are extensions of static web page files, and the extensions can be replaced with each other without causing errors (this refers to opening, but for a link, if it points to an html file and that html file is changed to an html file, then the connection cannot be found)
html loads faster than html
htm comes from the era of DOS three-character file extension
html supports multi-character extensions for the Windows era