Can javascript running inside an iframe affect the main page?

You can do it! Read here for more info

You can affect the document with contains the iframe by setting and getting variables from the window element:

// normally you use...
var whatever = "value";

// use the window object instead, which is shared 
// between the iframe and the parent
window.whatever = "value";

The other thing you should know is that you can access the main document via the parent object

inside the iframe you can use...

parent.someattr;

// or try this
parent.getElementById('some_element');

I think what you want is:

parent.getElementById('posts').innerHTML = 'why doth this faileth?';