I'd like to learn more about what I would call 'modular' web page elements, but I don't know what search terms to use. For example, I'd like to be able to have a navigation bar that isn't hard-coded in each page, but is loaded from another single source. I've read that PHP could insert code from a specified HTML file and I've seen some page elements, such as images, defined in CSS files and then called in the HTML file so I assume I could define an entire nav bar in a CSS element and then reference the element in the web page. I would rather use the CSS method, but whatever works is good. At this point, I don't know what to search for to figure out how to properly implement something like this. What is this called!?