Creating an iframe's child navigable as written is impossible.

[Psychpsyo]

What is the issue with the HTML Standard?

The create a child navigable algorithm tries to access members of a null object.
It does so by creating a new browsing context and document in step 3.
This proceeds to make active the document in step 23.
That algorithm, in step 3, tries to set document's visibility state to "document's node navigable's traversable navigable's system visibility state".
The node navigable is "the navigable whose active document is node's node document, or null if there is no such navigable."
Unfortunately, such a navigable will only be created in step 7 of create a child navigable, 4 steps after the creation of the document.
Thus, trying to access the document's node navigable's traversable navigable doesn't make any sense at this point in time.
Note that just not setting the visibility state here would cause the iframe's content document to not be considered visible.

EDIT: I am drafting a PR that'll hopefully be able to adress this.

[Psychpsyo]

Opened #12294 which should hopefully address this.