It's pretty clever. But adding load-bearing comments to any format is fraught, and this seems like no exception. I can see pros and cons, but let me focus this issue mostly on the cons, to provide a counterpoint to the current explainer.
If the idea is to reuse an existing format to take advantage of existing content, then there might be compat concerns with that existing content. Or, concerns about the existing software/users that consume text/uri-list content, being confused by all these page-collections-oriented comments showing up in their ecosystem.
On the other hand, if you think that there is likely not enough existing content out there for compat concerns to manifest, or enough of an ecosystem out there to be confused by these new comments... then is the text/uri-list format really worth building on in the first place?
The requirement that the extended text/uri-list format be isomorphically encodable in a uri-list [sic] scheme is also somewhat restrictive. Although the ? in URL <-> #? mapping is pretty close, the one envisaged in https://github.com/bokand/page-collection#hierarchical-lists is not at all close.
An alternative path that might be worth investigating is a custom format which does not have existing content out there to cause compat and ecosystem concerns. This format could have first-class support for extensibility of the type you're envisioning, and could even be designed from the beginning to be isomorphic to a corresponding URL scheme.
It's pretty clever. But adding load-bearing comments to any format is fraught, and this seems like no exception. I can see pros and cons, but let me focus this issue mostly on the cons, to provide a counterpoint to the current explainer.
If the idea is to reuse an existing format to take advantage of existing content, then there might be compat concerns with that existing content. Or, concerns about the existing software/users that consume text/uri-list content, being confused by all these page-collections-oriented comments showing up in their ecosystem.
On the other hand, if you think that there is likely not enough existing content out there for compat concerns to manifest, or enough of an ecosystem out there to be confused by these new comments... then is the text/uri-list format really worth building on in the first place?
The requirement that the extended text/uri-list format be isomorphically encodable in a
uri-list[sic] scheme is also somewhat restrictive. Although the?in URL <->#?mapping is pretty close, the one envisaged in https://github.com/bokand/page-collection#hierarchical-lists is not at all close.An alternative path that might be worth investigating is a custom format which does not have existing content out there to cause compat and ecosystem concerns. This format could have first-class support for extensibility of the type you're envisioning, and could even be designed from the beginning to be isomorphic to a corresponding URL scheme.