> The browser has to download the entire document to construct the DOM Actually, the browser can construct the DOM for the first part of the page while it's wa

> The browser has to download the entire document to construct the DOM Actually, the browser can construct the DOM for the first part of the page while it's waiting for the rest of it to download. Like, if the browser starts downloading a page and it sees this: <body> <h1>My Cool Website</h1> <p>Hello there ...it can add the <h1> element to the DOM, since no matter what comes after this in the HTML code, the <h1> will always be first on the page. (It might actually be able to add the <p> tag, too. I'm not 100% on the details.) A cool demo of this is https://harmless.herokuapp.com/main https://harmless.herokuapp.com/main , a JavaScript-less chat app that works by holding the connection open (essentially never "finishing the page load") and sending new chat messages as they arrive. > CSS in the head tag all has to be downloaded and parsed into the CSSOM before the browser can make smart decisions about what to render. That's true, but in this case, the CSS is already present in the browser cache, so it doesn't need to be re-downloaded.