In the early days of the Web, one solution to the repeated identical content download problem was frames. If you're too new to web development to remember, frames presented a way to break a single-page view into several different HTML files—navigating through the site involved reloading one or more of the frames while the others stayed the same. Frames helped a website to load faster and made a site easier to maintain, but in the end, they created more problems than they solved. Framed websites were easily broken, were difficult for search engines to index, often broke the back and forward buttons, and made it difficult or impossible for the site visitors to bookmark pages, share links, or print content. Because of all these problems, the use of frames has fallen out of favor.
More recently, single-page applications have started to become more popular. If you log into your Twitter account and start clicking around, you'll notice that the whole page refreshes...