This is best described on the spec doc - please read it!
Specification: Render library specification
Forum discussion: https://moodle.org/mod/forum/discuss.php?d=261202
Advisory posts in General developers forum and Theme developers forum
Please do not discuss the specification here. We would like all feedback to come through the forums, with everything in one place it will be easier to collate the discussions that go on and to produce the final spec.
Note - this is the second epic - designed to capture tasks that should be done at a later date than the initial set of tasks (undetermined).