Problem mixing a scene with video clips in the VSE I am using Blender 2.79 on a fully updated iMac.

I want to create a 2-D animation and have 2 video clips before this animation.

The issue is that even if the clips are above the scene strip, or even as in the screen-shot, the scene starts after the videos, the videos do not render.

Strangely, they are show in the preview, but not in the rendered video or even image (F12), as you can see in the screen shot.

Am I doing something wrong, or is it simply not possible in the Blender VSE to mix video clips with the scene?

I could render the scene and then import the video into another blender file, but then I would have to render the movie twice and rendering time in Blender is already quite long.

Thanks for your kind feedback.
The main output of Blender is the 3D window, after that you must activate the Compositor to apply effects (like color grade or blur) by checking the Compositor box in the Scene Properties panel. Then you can bring that compositor output (or the 3D scene) into the VSE for editing with other footage (as you have done). But for this render engine to work you must check the Sequencer (as in Video Sequence Editor) box. 

Note that you can check either or both the Compositor or the Sequencer box, depending on what functions you want.

If you only want to edit video clips or pre-rendered animations, then you MUST check the Sequencer box, otherwise the VSE output will not be rendered. In fact all you will see is what is present in the 3D view.