You have to use PHP.

Made your index.php instead of index.html. you can make header.php and footer.php, then call them from index.php. So the code in header.php willlbe placed in index.php and will be processed and displayed.

<?php

$html_title = "Home";
$html_keywords = "academy, insa, fosma, massa, nigdi, pune, merchant navy, indian navy, voluntary youth, nda, army, air force, coast guard, marine, para military forces";
$html_description = "Academy";

include("./header.php");

?>
The html code will follow and then you have to call footer.php as

<?php
include("./footer.php");

?>