For many sites, especially content sites (typical blogs, brochure, and documentation sites), very little JavaScript is actually needed. In fact, it will often be found in isolated corners of the page. Examples are an interactive image carousel, share buttons or a hamburger menu. Hydration is the process of converting the HTML for these little interactive enclaves and making their state consistent. In fact, it is only these islands which require hydration, the rest of the page will just be plain HTML and CSS. That said, traditionally, servers shipped JavaScript for hydrating the entire page and not just the little sections where it is actually needed. Partial hydration and islands architecture are a smarter, more modern approach. We ship zero JavaScript by default, and only ship JavaScript for the islands (which opt-in to hydration).