Thanks to Ulrich for the tooltip example.
For me a killer feature that I need right now is to display element Notes in a tooltip in the HTML simulation.
I am prototyping CRM customisations, and I store meta data on each field in the Notes which is extracted later by Looksie (http://www.looksie.me) to automate the creation of specifications and later even code.
Using Ulrich’s tooltip, I could then just have a generic Mouse Over and Mouse Out that updates the tooltip text from the element Notes.
I realise you would might want to optimise the HTML generation to only includes Notes that were referenced..