You have used HTML to add stuff to a page.  You have used CSS to make it look a certain way.  JavaScript can change either of those after a page has loaded.  If you want a page to change after it has loaded use JavaScript.  It can make slide shows, enlarge or shrink things, reveal or hide things and much more.  Here you get a very simple introduction just so you see it working.

Before you start you made a copy of your Web folder before you played around with the CSS.  Make another copy and call it musichtmlcss or similar.  You now have the original folder to work on with JavaScript and two copies as back ups in case something goes wrong.