Chapter 10. Brackets

Since the early 2000s, the web community has executed the largest format migration in its history: XML to JSON. Two decades of rewritten APIs, retired toolchains, retrained developers. And what changed?

1999	2019
<person><name>Ada</name></person>	{"person":{"name":"Ada"}}

Twenty years of progress. The book’s only sarcastic figure caption; it has earned it.

There is a term for activity of this shape: lateral churn — motion that looks like innovation but isn’t. Real innovation is vertical: new layers, new semantics, new abstractions on top of what already works. That is how the web was designed to grow — and Parts II and III have shown, formally, that the vertical direction was open the entire time.

So far, that is a story. The apparatus can measure it. The claim to check: XML→JSON was lateral — a change of syntax presented as a change of substance. The instrument: the seven properties.

Both formats are one model. An XML document and a JSON document are ordered labeled trees; their differences — attributes versus members, elements versus arrays — decorate the same structure. Chapter 5’s requirements apply to the structure, so the scores transfer wherever a requirement sees only the tree. R3 is the exception, and the formats part ways there.

R2. Trees have no coordination-free merge. Two JSON documents have no defined composition at all. Concatenation is not valid JSON, and “deep merge” is a per-application policy — which key wins, whether arrays append or replace. And a policy shared between parties is coordination. In the terms of Appendix B, meaning lives in arrangement (position, nesting, order). That rejects B-2d (atomicity: a state says exactly what its atoms say). So the representation lemma (B.1) — the proof behind the union law — never gets started. ✗

R3. JSON has no reference type. A URL in a JSON string is a string; the format’s specifications define grammar and leave interpretation to applications, so nothing distinguishes a link from a postcode. XML held fragments of the property. Namespaces gave vocabulary terms global names; xml:id and XLink offered standardized reference, largely unused. The migration shed the fragments too. AWWW §4.4 named three good practices in 2004 — link identification, Web-wide linking, hypertext links — and all three fail in the format the industry migrated to. ✗ for the destination; the origin’s fragments salvage a ~.

The tooling deficit, itemized.

capability	XML stack	JSON stack
schema	XSD (XML Schema), 2001	JSON Schema — drafts since 2010, still a draft
query	XPath, 1999	JSONPath — RFC 9535, 2024
transformation	XSLT, 1999	—
intra-document addressing	fragments + XPointer	JSON Pointer — RFC 6901, 2013
vocabulary scoping	Namespaces, 1999	—

The right column arrived a quarter century late where it arrived at all: a standardized query language in 2024, twenty-five years after XPath; transformation and vocabulary scoping with no entry. The migration’s tooling deficit is two decades old and still open. And longevity ran the other way. The JSON column’s empty transformation cell is filled by whatever framework is current, and most JavaScript frameworks of 2010 have already been retired. XSLT — frozen at its 1999 revision, as Chapter 9 filed — still runs in every browser as of this writing.

Column: XML stack.

	XML stack
R1	✓ — trees encode any domain
R2	✗ — no coordination-free merge; meaning lives in arrangement
R3	~ — namespaces, xml:id, XLink: standardized fragments, largely unused
S1	~ — vocabulary separated by schema; arrangement and data still fuse
S2	✓ — XPath and XSLT: query and transformation with specified semantics
S3	✓ — one standardized language per capability; components substitute
S4	~ — fragments and XPointer address into documents, largely unused

The S-properties of the deployment style — REST as practiced. The style’s genuine inheritance from HTTP survives in the scores: resources carry URIs, so S4 earns partial credit at the resource grain. The credit stops there, because values inside a representation cannot link onward, so the data ends at every document boundary. The style’s own definition requires hypermedia links (Fielding 2000, §5.1.5); the deployments that adopted the style’s name discarded the requirement. Query and transformation semantics are implementation-defined (S2 ✗). Substituting a component means renegotiating a bespoke contract per pair of parties (S3 ~). Every payload ships arrangement and data fused (S1 ~ — the endpoint separates, the representation does not).

Column: JSON/REST.

	JSON/REST
R1	✓ — trees encode any domain
R2	✗ — no defined composition; merge is per-application policy
R3	✗ — no reference type; links are strings
S1	~ — endpoints separate; representations fuse arrangement and data
S2	✗ — interpretation left to applications; query standardized 2024, transformation never
S3	~ — substitution behind bespoke contracts
S4	~ — resources have URIs; values do not link onward

The opening question — and what changed? — now has a measured answer: the migration changed brackets, shed tooling, and gained not one property. The cells that moved — R3, S2, S3 — moved down. Lateral, by the numbers.