How to remove an HTML element using Javascript?

What’s happening is that the form is getting submitted, and so the page is being refreshed (with its original content). You’re handling the click event on a submit button.

If you want to remove the element and not submit the form, handle the submit event on the form instead, and return false from your handler:

HTML:

<form  onsubmit="return removeDummy(); ">
    <input type="submit" value="Remove DUMMY"/>
</form>

JavaScript:

function removeDummy() {
    var elem = document.getElementById('dummy');
    elem.parentNode.removeChild(elem);
    return false;
}

But you don’t need (or want) a form for that at all, not if its sole purpose is to remove the dummy div. Instead:

HTML:

<input type="button" value="Remove DUMMY" onclick="removeDummy()" />

JavaScript:

function removeDummy() {
    var elem = document.getElementById('dummy');
    elem.parentNode.removeChild(elem);
    return false;
}

---

However, that style of setting up event handlers is old-fashioned. You seem to have good instincts in that your JavaScript code is in its own file and such. The next step is to take it further and avoid using onXYZ attributes for hooking up event handlers. Instead, in your JavaScript, you can hook them up with the newer (circa year 2000) way instead:

HTML:

<input id='btnRemoveDummy' type="button" value="Remove DUMMY"/>

JavaScript:

function removeDummy() {
    var elem = document.getElementById('dummy');
    elem.parentNode.removeChild(elem);
    return false;
}
function pageInit() {
    // Hook up the "remove dummy" button
    var btn = document.getElementById('btnRemoveDummy');
    if (btn.addEventListener) {
        // DOM2 standard
        btn.addEventListener('click', removeDummy, false);
    }
    else if (btn.attachEvent) {
        // IE (IE9 finally supports the above, though)
        btn.attachEvent('onclick', removeDummy);
    }
    else {
        // Really old or non-standard browser, try DOM0
        btn.onclick = removeDummy;
    }
}

…then call pageInit(); from a script tag at the very end of your page body (just before the closing </body> tag), or from within the window load event, though that happens very late in the page load cycle and so usually isn’t good for hooking up event handlers (it happens after all images have finally loaded, for instance).

Note that I’ve had to put in some handling to deal with browser differences. You’ll probably want a function for hooking up events so you don’t have to repeat that logic every time. Or consider using a library like jQuery, Prototype, YUI, Closure, or any of several others to smooth over those browser differences for you. It’s very important to understand the underlying stuff going on, both in terms of JavaScript fundamentals and DOM fundamentals, but libraries deal with a lot of inconsistencies, and also provide a lot of handy utilities — like a means of hooking up event handlers that deals with browser differences. Most of them also provide a way to set up a function (like pageInit) to run as soon as the DOM is ready to be manipulated, long before window load fires.