This is what I was doing in 2015 when my brain was fried with HTML and CSS and I was desperate to avoid having to learn even more syntax (PHP). I remember trying something like .shtml and .dhtml.

It didn't feel "real". I had never seen web pages with those extensions. Frankly, I hadn't even seen .html pages in several years because everybody edits the .htaccess file to prettify the URLs.

I ended up biting the bullet and going with PHP. Surprised to learn that there still isn't anything out of the box that handles this.