As of March 2011, this is a trick question – NO mobile browser even attempts to support contentEditable/designMode (aka WYSIWYG, rich HTML textareas) on portable devices like cellphones, smartphones, iPad, iTouch, etc.
Desktop browsers have been supporting it since IE 5.5 in July 2000 over a decade ago. The Mozilla 1.3 (Gecko) engine implemented it in late 2002, so Firefox 1.5 had it in May 2003.
Also see: http://caniuse.com/contenteditable
So which one of these will be the first mobile browsers to have it?
Microsoft IE for Mobile
Google Android (Chrome/webkit)
Apple Safari Mobile (webkit)
Firefox Mobile (Fennec)
However I have no information on the brand new Internet Explorer Mobile 9 (aka IE9 Mobile) which was only demonstrated February 2011 by Microsoft.
Since Microsoft invented content-editable, there is a chance they ported it into the mobile IE9 from their newest Trident engine. If anyone can confirm or deny this, please let me know? Thanks!