After a lot of discussion and testing, we decided to adopt the Aloha Editor as the WYSIWYG editor for Spark, and possibly for Drupal 8 core. Check out Wim's blog for the details. In short, it is the best HTML5 based WYSIWYG editor; fast, well-written and future-proof.
Here is a screenshot of our latest designs of how we envision integrating the Aloha Editor in Spark on a mobile device:
I also wanted to give a big thank you to Haymo Meran, creator of Aloha Editor and Director of Product Experience at Gentics Software GmbH, both for hosting a Drupal/Aloha collaboration sprint at his company's offices in Vienna last month, and also for changing the license of Aloha Editor to GPLv2+ (from AGPLv3) so that it could be used with Drupal!
It's an incredible gift to the Drupal project and the Open Source community at large. Thanks!