textarea fields into an editor and the editor is capable of all the major actions (text-styling, adding image/file/video/tables/lists).
The uploading of files and images makes use of HTML5 APIs and they can be uploaded via drag 'n' drops besides selecting files.
Redactor can work inline or fullscreen, there are many settings to customize it and it is multilingual (already supports many languages).
P.S. The website of the resource is half English and half Russian but it is coded in English and demos + docs help understanding how it works.