During every release cycle we have always added a preliminary area to help.ubuntu.com where we post the pre-release documentation for that cycle. We have just added a permanent "development" section to help.ubuntu.com, where we will always have the yet to be released desktop help and Ubuntu serverguide works in progress.
For example, and currently, the 17.10 Desktop help HTML is there, as is the 16.04 Ubuntu Serverguide (us English only), with a couple of yet to be officially translated and published changes. Note that the severguide is only published on LTS cycles, which is why 16.04 is still the master publication.