Javascript import html is it possible?

As above, one method is to use jQuery load. I happened to be doing the exact same thing now, so will post a quick example.

Using jQuery:

$("#yourDiv").load('readHtmlFromHere.html #readMe');

And your readHtmlFromHere.html page would consist of:

<div><div id="readMe"><p>I'm some text</p></div></div>

Here's how you could use just javascript to add a footer to your page.

var ajax = new XMLHttpRequest();
ajax.open("GET", "footer.htm", false);
ajax.send();
document.body.innerHTML += ajax.responseText;

You can use ajax to return a whole HTML page. If you wanted to replace the whole page you could replace the body tag and all it's children currently on the page with the body tag returned from the ajax call.

If you wanted to just replace a section you'd have to write a server-side script to create that section, then use ajax as above but just replace an element rather than the whole page.