Broken: 2.79 5bd8ac9
Short description of error
When rendering an animation to the ffmpeg preset output (from VSE and compositor), the RGB button is not auto selected on the output panel.
This can result in an animation that does not have premultiply process on alpha images. Output looks aliased if using black and white keyed text.
Exact steps for others to reproduce the error
Use a .png with RGBA of a white word with alpha background. Add this to VSE timeline as a strip. (a test image is packed with the test .blend file)
Go to Properties window and choose ffmpeg on the Output panel. **NB: that RGB is not selected nor is BW.
Press f12 to render the animation.
Rendered file looks aliased because premultiply was not applied to the alpha image on the timeline.
If you do the same with the compositor you must explicitly ADD the "Convert Alpha" node, otherwise it will create "Straight" alpha images.
The VSE does not give you an explicit option to create Straight vs Premult, so the RGB should be active when you choose ffmpeg as an output.