At Collabora we have been working on a tech demo of a simple video editor application on top of WebKit and HTML5. It is a very simple editor powered by GStreamer Editing Services that allows you to drag-n-drop video clips and images to a timeline for editing.
The support for transitions is already merged in the editor’s git master. It still has a few problems that we are currently working on, but it gives an idea of the possibilites we get by adding video editing capabilities to WebKit.
You can find instructions on how to try the application in a blog post by Mateu Batle about the application.