If an image fails to load, the browser draws a little box with some alternate text describing that box. If the CSS doesn't load, your text and content is displayed in a weird font without the grid layout you were using, but if you wrote your HTML semantically (using <h1> instead of <div class="title"> etc.), the browser can still show most of your content, and you can still move around on the page.

If the JavaScript fails to load and you were using it to significantly alter the content on your page, for example loading a news article asynchronously, the entire page fails to load.

I don't mean to pick on this app in particular (I actually think it's really cool and I plan on using it and learning from it), but take a look at what happens to http://hswolff.github.io/hn-ng2/ when you switch off JavaScript--it's completely unusable. Now try switching off JavaScript on Hacker News--all the links and comments are still there.