I would  avoid mixing js and html.
Though this could be encapsulated and given a more semantic meaning. In fact, just reading this is hard to guess why this is needed.

```javascript
function Application () {
   this.markJsAvailable = function () {
     $('body').addClass('js');
   };
  this.initialize = function () {
    this.markJsAvailable();
    // and other code initialization
  }
  this.initialize();
}

$(document).ready(function () {
  new Application();
})();
}
```

Or something along these lines.
But there might be reasons that i'm not aware of that make the afford mentioned better.