whats the difference of
example.html
and
example.htm
?

One has an L.
One has no L.

It goes back to the 8/3 file names.
Either will work.
Some site providers like Angelfire require your home page to use .html.

8/3 file names?
and if either will work, why didnt they just leave it htm?

Because using just 1 extension name wouldn't work. That would be too simple, that's not the way the world works.

Long ago in the land of DOS & Windows 3.whatever,computers had such limited memory,it was decreed that file names should consist of 8 characters to the left of the DOT & 3 characters to the right of the DOT.

That has changed.
Now file names can be bigger.
It is just a legacy from the past.

makes more sense, thanks!

Ya, with DOS and Windows 3.x series, the filenames was in 8.3 format. 8 characters for the name of the file, and a dot and an addiotional 3 characters for the file extension. The file extension couldnt be any longer than 3 characters.
Still today most file extensions are 3 letter long, and .html was shortened to .htm
But nowadays, most websites use .html

Then Windows 95 came and the problem was no longer, because it has LFN (Long FileName support). Unix-like system's have had long filename support forever though, I think.

So technically there is no difference, its two extensions, but its the same file and they get parsed and processed in the same way. You can use whichever of them you like, I did a Google search with filetype: and .html was more widely in use than .htm

Yeah Windows 9x implemented LFN, however there's still an MS-DOS filename attached to longer filenames so that they can be accessed in MS-DOS mode on the OSes.

For example:
Windows: C:\Program Files\CCleaner\CCleaner.exe
MS-DOS: C:\Progra~1\CCleaner\CCleaner.exe

Strangely some programs that have an autostart parameter in the Run section of the registry still utilize the MS-DOS short name.

Redundancy, I guess.
Making sure their software will work with older machines.
Although I doubt games like Doom would use squiggles