I tried in Chromium on Ubuntu, and Firefox and IE on Windows. It looks trainwreck-y on all of them for me.

By the process of elimination, I believe it's because I'm using a T-mobile wireless dongle. The upstream proxy attempts to reduce bandwidth by doing things like coalescing external CSS and JS inline. I've seen this break one other site (LinkedIn profile editing, which seemed to be a bit fast and loose about its use of ajax) but every other site I've used has been fine (even e.g. Asana, which is rather intensive.)

I looked at the source of codehs.com (insofar as I can figure it out, what with all the CSS and JS put inline...) and I'll change my actionable feedback to:

Try to keep it simple.

That seems like far more stuff than you'd actually need on a landing page.

I also noticed that, when loading on IE, it pops up a message saying:

    You are using Internet Explorer, an unsupported browser :(
    You should download Google Chrome so you can get started!

It's a pity that the site doesn't support the browser with the largest market share -- that's probably something you should work on.

It does. Google Chrome has the largest market share.

Thanks for the feedback. These are all things on our to-do list.