When the web was young, HTML pages were served to clients running web browser software that would turn the HTML text response into rendered pixels on the screen. At first these were static HTML files, but then things like PHP and others came along to allow the server to customize the HTML sent to each client.

CSS came along to change the appearance of what got rendered. JavaScript came along to make the page interactive. Suddenly the page was no longer the atomic unit of the web experience: pages could modify themselves right there inside the browser, without the server being in the loop at all.

This was good because the network is slow and less than 100% reliable. It heralded a new golden age for the web. Progressively, less and less of the HTML content was sent to clients as pre-rendered HTML, and more and more was sent as JSON data that the client would render into HTML using JavaScript.

This all required a lot more work to be done on the client, though, which meant the client had to download a lot more JavaScript. Before long we were shipping MEGABYTES of JavaScript down to the web browser, and we lost the speediness we had gained by not reloading the whole page all the time. Page transitions were fast, but the initial load was slow. Megabytes of code shipped to the browser can multiply into hundreds of megabytes of device memory consumed, and not every device is your state of the art Macbook Pro.

Single Page Applications ultimately do the same thing as that old PHP application did - render a bunch of HTML and pass it to the browser to render. The actual rendered output is often a few kilobytes of plain text HTML, but we downloaded, parsed and executed megabytes of JavaScript to generate those few kilobytes of HTML. What if there was a way we could keep the interactivity of a SPA, but only send the HTML that needs to be rendered to the client?