Traditionally, static site generators sent only HTML to the browser initially to give it a fast feel. The next phase of loading involved getting the JavaScript in to make the page interactive. This is the hydration step. This has inefficiencies since many parts of a page may have no JavaScript at all, yet are still “hydrated” along with the parts which actually need hydrating. A downside is extra JavaScript code gets sent to the browser to handle this. With partial hydration, you can control which parts of the page get hydrated, reducing the amount of JavaScript that has to be sent to manage hydration and making the page faster overall. You can even ship a page with no JavaScript at all. By default, Astro ships no JavaScript and you only hydrate what you want hydrating.