The HD DVD specification (listed here, also here) can no longer be obtained from the DVD forum, given that the manufacturing of HD DVD was discontinued a long time ago. Before that, the specification was paywalled, as all of their specification books are, so there is zero chance of finding it in web archives.
I believe the specification of HD DVD should not be lost to history. It would be disappointing to see the specification of an entire data storage format disappear.
Since HD DVD is a discontinued format and no longer licensed by the DVD forum, publishing the specification would not hurt the profits of the DVD forum. I don't even think the DVD forum will exist by 2040. Then, the rest of the specifications will also likely become lost media.
I am posting this here for the slight chance that someone out there who possesses a copy of the HD DVD specification or works at a company that does is gracious enough to make it available to the public.
If you don't have the means to make it public right away, please at least consider privately mailing it to various tech journalists so at least one copy outlives the DVD forum. Thank you.