Going back to first principles, what does a URL do? It identifies a unique resource. You can link to it directly; you can bookmark it; search engines can index it; you can copy and paste it and email it to someone else, who can click it and end up seeing the same resource you saw originally. These are all excellent qualities. URLs matter.

So we want unique resources to have unique URLs. But at the same time, browsers have always had a fundamental limitation: if you change the URL, even through script, it triggers a roundtrip to the remote web server and a full page refresh. This takes time and resources, and it seems especially wasteful when you are navigating to a page that is substantially similar to the current page. Everything on the new page gets downloaded, even the parts that are exactly the same as the current page. There is no way tell a browser to change the URL but only download half a page.

The HTML5 history API lets you do this. Instead of triggering a full page refresh, you can use script to, in essence, download half a page. This illusion is tricky to pull off, and it requires some work on your part. Are you watching closely?