HTML is a Hyper Text Markup Language…
HTML file is a text file small markup tags.
You Want Try It…
<html>
<head>
<title>This is text </title>
</head>
<body>
This is my first text. <b>This text is bold</b>
</body>
.html and .htm are the extensions.
1. “.htm ”
This was started in the early days say 16bit OS level where the name of the file and extension are limited to 8 chars and 3 chars respectively. The 3 char extension says more about the version and its compatibility including javascript and layout rendering between IE and NETSCAPE.
2. “.html”
As the 16bit OS became the 32bit and limitations of extension was not a barrier. the W3C came to a conclusion on putting the HyperTextMarkupLanguage *HTML” as a valid extension for all the static client side web pages. Here the version, compatibility and support increased and it started to become more visually meaningful.
If you look from now and go back to the previous OS version “.html” is not recognized as a valid extension and it will not work as desired. For current OSs which they are 64/32 bit the previous compatibility (which I meant as support) exists and .html and .htm are like the web pages with new extension format and old one, but in content rendering and stuff are same.
There are some applications/software where “.htm” is used and recognized. For current gen apps “.html” valid and can be used without any problem.