R – Prototype getStyle(‘height’) function

Note: most current browsers support HTML <template> elements, which provide a more reliable way of turning creating elements from strings. See Mark Amery's answer below for details.

For older browsers, and node/jsdom: (which doesn't yet support <template> elements at the time of writing), use the following method. It's the same thing the libraries use to do to get DOM elements from an HTML string (with some extra work for IE to work around bugs with its implementation of innerHTML):

function createElementFromHTML(htmlString) {
  var div = document.createElement('div');
  div.innerHTML = htmlString.trim();

  // Change this to div.childNodes to support multiple top-level nodes
  return div.firstChild; 
}

Note that unlike HTML templates this won't work for some elements that cannot legally be children of a <div>, such as <td>s.

If you're already using a library, I would recommend you stick to the library-approved method of creating elements from HTML strings:

• Prototype has this feature built-into its update() method.
• jQuery has it implemented in its jQuery(html) and jQuery.parseHTML methods.

When I used prototype i'm sure I did something like this.

var firstElement = Form.findFirstElement(document.forms[0]);
if(firstElement !=null)
    firstElement.activate();

I would substitute your form for document.forms[0]

Prototype also has the method of focusFirstElement on the Form class, but I couldn't use that because some pages I would load didn't have forms. You might be able to just call Form.focusFirstElement($(form)); or a similar variation.

Edit:
I thought you could use the Form extensions on different types of elements but it doesn't appear to work, however you can take something from Form and apply it directly. The following code should work as you intend.

var firstElement = Form.getElements($("parentDiv")).find(function(element) {
    return element.type != 'hidden' && !element.disabled &&
    ['input', 'select', 'textarea'].include(element.tagName.toLowerCase());
});
if(firstElement != null)
    firstElement.activate();