Then we need a way to set the base URL of a document from another
specification. When set, this changes the URL of the document and then
for each inclusive descendant, in tree order, notifies the node of a
base URL change, if the node registered for base URL changes. This
allows e.g., `<a>` to correct its URL.
We also change adopt to run these base URL change notifications. It's
unclear whether they should run before the adoption notification or
after, or whether the adoption notification by itself is sufficient. I
suspect we want to have both separately so you don't forget any.
@ajklein @coonsta @smaug---- does this make sense to you?