Won't that mean painting ugly unstyled content? Yes it will, but that's better than making the user wait forever on a blank screen. For example, on an example page, it would look like this:

(Try your browser here)

Displaying only the html isn't a new thing: text-based browsers have done this for years and there's already one for Android. But the user shouldn't have to think "this page might be slow because I have a bad internet connection and it's really big so let's open it in a text-only browser". Instead regular browsers should just paint what they've already downloaded if they've been keeping the user waiting too long.

Update 2013-07-11: Instead of a pure timeout, it might be better to display a button like "unstyled text available now, click here to load" that would force an early paint. You would still want a timeout of a few seconds so that you don't display this button when someone would get the whole page if they were just a little more patient.

[1] What your browser does to download a simple web page:

1. Use DNS to look up the IP of your site:

         Browser:    What is www.jefftk.com?
         DNS Server: www.jefftk.com is 209.20.65.89

2. Open a TCP connection to the site.

         Browser:    SYN
         Web Server: SYN-ACK
         Browser:    ACK

3. Download the HTML

         Browser:    GET / HTTP/1.1
                     Host: www.jefftk.com
         Web Server: Server: nginx/1.4.1
                     Date: Thu, 11 Jul 2013 11:27:07 GMT
                     Content-Type: text/html
                     ...
                     <link rel=stylesheet href=main.css>
                     <script src=scriptes/main.js<
                     ...
                     about a month ago I &lta href="news/2013-06-05"&gt
                     got some citric acid&lt/a&gt. Since then I've tried
                     putting it on and in various things. Chocolate was
                     strange, Strawberries were good, but my favorite has
                     definitely been watermelon:
                     <p>
                     <img src="/citric-acid-watermelon.jpg">
                     ...

4. The browser could paint here (or continuously as it downloads the html) but doesn't, to avoid a flash of unstyled content.
5. Download the referenced resources that go are required for displaying the page:

         Browser:    GET /main.css HTTP/1.1
                     Host: www.jefftk.com
         Web Server: [contents of main.css]
         Browser:    GET /scripts/main.js HTTP/1.1
                     Host: www.jefftk.com
         Web Server: [contents of /scripts/main.js]

6. Display the page in its initial state (first paint!)
7. Download lower priority resources:

         Browser:    GET /citric-acid-watermelon.jpg HTTP/1.1
                     Host: www.jefftk.com
         Web Server: [contents of /citric-acid-watermelon.jpg]

8. Display the page in its final state. There could be multiple paints during the previous step.