Why the browser read only HTML files..or any others there..?

Answer 1

Hello, Gokul S !
Most of the pages on the World Wide Web are written in computer languages ​​(for example, HTML) that allow web authors to structure text, add multimedia content, and specify what kind of appearance or style the result should have.

As for each language, they have their own grammar, vocabulary and syntax, and every document written in these computer languages ​​must follow these rules. Languages ​​(X) HTML for all versions prior to XHTML 1.1, use machine-readable grammars, called DTDs, a mechanism inherited from SGML.

Answer 2

Thank You Sir
Alexander Sokolov