There are no open source alternatives for the wysiwyg editor part of optimizely. But before you roll your own backend implementation have a look at https://face

There are no open source alternatives for the wysiwyg editor part of optimizely. But before you roll your own backend implementation have a look at https://facebook.github.io/planout/ https://facebook.github.io/planout/ . (We had success in combining planout with a cms for the visual part. Marketing could do simple a/b tests. More complicated tests where done in the backend together with a developer.)