If I have the same header/sidebar/footer for multiple html pages, is there a way to make it so that I don't have to change every single page each time?
so you will include in html pages header and footer like this way <!--#include file="header.html" --> <!--#include file="footer.html" -->
That works in some cases, but not in all cases. (It depends on the server configuration, for the most part.) Including the file in server-side code works in all cases. Use include('theFileName.html'); in a PHP file that has all your other html in it. There doesn't have to be any actual PHP code other than the <?php and ?> tags and the include() files. The rest can be HTML, CSS, Javascript - whatever you have now.