Drupal 7 live preview module
I've given this problem some thought (actually began writing code for a D7 live preview), and as it happens I simply don't think live preview is that simple in Drupal.

Live preview on SE.sites is efficient because there are no AJAX requests going on for rendering the preview. In a nutshell SE.sites merely clone the text from the editor textarea into the preview div, where CSS classes are applied to it. In reality there is more processing happening in the background as the text that you enter must be converted into real markup (ie, when using lists).

In Drupal however there can be a lot of pre-processing of text, depending on what modules you have enabled. For instance, take the &lt;!--break--&gt; control sequence (which is part of core behavior, btw). Does that really mean something in HTML? No, it must be preprocessed by Drupal into teaser and full body text snippets; take the CKEditor and the IMCE modules for example, they also depend on preprocessing text to translate special markup for images into actual &lt;img&gt; tags.

What that means is that in order to have a live preview that is faithful to it's final state, you must run whatever text you enter through a number of hooks, preprocessors, renderers and what have you. Else your live preview will not represent the final product. Therefore, having a faithful Live Preview in Drupal would mean sending an AJAX request to the back-end every time the editor in the front-end is updated. And then, you can ask yourself the question? Is this scalable? That would probably depend on your use case. If you want Live Preview for a site with a big editorial team (think www.nowpublic.com, a Drupal site), the answer would probably be no. But maybe if it is for a small site, with a few visitors or editors, then the answer would be maybe, but it still raises performance concerns.

So taking those points into consideration, what kind of preview would you be looking for? Slow &amp; Faithful or Fast and Unfaithful?