SSI File vs PHP Includes

Must all my HTML pages be converted to PHP for SSI includes to work in them?

I’ve created a page with a header that includes a navbar and moved it to an include file (incl-header.html) and selected SSI File as the type. I can import it into my existing html files and the include displays nicely in Design View. But when I open the converted pages in a browser, the include does not display. In code view, the include line is commented out. So am I doing something wrong or do I ned to covert to PHP?
Here is what is placed on the page:

<!--#include file="incl-header.html" --> …does not display the include

If I convert the page to a PHP and select PHP Include as the type, the include displays on the page in the browser and works well:

<?php include 'incl-header.html';?> …displays the include