IBM has released an online HTML5 editing tool called Maqetta, hosted by the Dojo Foundation. Maqetta is an open source project that provides WYSIWYG visual authoring of HTML5 user interfaces. The Maqetta application itself is authored in HTML, and therefore runs in any HTML 5 browser (sorry Internet Explorer is not included [LOL]) without requiring additional plugins or downloads.

From the Maqetta website;

Some Maqetta features include:

• a WYSIWYG visual page editor for drawing out user interfaces
• drag/drop mobile UI authoring within an exact-dimension device silhouette, such as the silhouette of an iPhone
• simultaneous editing in either design or source views
• deep support for CSS styling (the applications includes a full CSS parser/modeler)
• a mechanism for organizing a UI prototype into a series of "application states" (aka "screens" or "panels") which allows a UI design to define interactivity without programming
• a web-based review and commenting feature where the author can submit a live UI mockup for review by his team members
• a "wireframing" feature that allows UI designers to create UI proposals that have a hand-drawn look
• a theme editor for customizing the visual styling of a collection of widgets
• export options that allow for smooth hand-off of the UI mockups into leading developer tools such as Eclipse
• Maqetta's code base has a toolkit-independent architecture that allows for plugging in arbitrary widget libraries and CSS themes

I'm currently testing Maqetta and will blog more about my finding at a later time.