Related: Network Optimization for SPAs and React Apps covers the modern optimization techniques that work with HTTP/2 rather than around HTTP/1.1's limitations.

In 2012, a frontend developer optimizing a website's network performance would combine all their JavaScript into one file, combine all their CSS, put icons into a single sprite image, split their static assets across multiple subdomains, and inline small images as base64 strings in CSS. All of these were correct. They were the right things to do.

By 2020, most of them were wrong. Not slightly suboptimal; actively counterproductive on HTTP/2.…