Fix animation in Chrome 34 and Firefox 26

This may look hacky, but it is required to make the animation work in the new Chrome version, at least on Linux (I'll run a test on Windows later).

What it does is add

```html
<style>
:not(.animated).box {
  animation-name: none;
}
</style>
```

to the `<head>`. It is required because otherwise the animation gets fast-forwarded by the browser (presumably because `visibility` is set to `hidden` by the plugin).