Draft for freezing the HTML simple repository API

Signed-off-by: William Woodruff <william@astral.sh>

Lint

Signed-off-by: William Woodruff <william@astral.sh>

More lint

Signed-off-by: William Woodruff <william@astral.sh>

Deprecation -> freezing

Signed-off-by: William Woodruff <william@astral.sh>

Mention optimization problems

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Tweak conneg language

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Update spon/delegate

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Precision

Signed-off-by: William Woodruff <william@astral.sh>

Author: woodruffw

peps/pep-9999.rst

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+PEP: 9999
+Title: Freezing the HTML simple repository API
+Author: William Woodruff <william@yossarian.net>
+Sponsor: Donald Stufft <donald@stufft.io>
+PEP-Delegate: Donald Stufft <donald@stufft.io>
+Discussions-To: Pending
+Status: Draft
+Type: Standards Track
+Topic: Packaging
+Created: 16-Apr-2026
+Post-History: `13-Apr-2026 <https://discuss.python.org/t/106959>`__
+
+
+Abstract
+========
+
+This PEP proposes freezing the
+:ref:`standard HTML representation <packaging:simple-repository-html-serialization>`
+of the simple repository API, as originally specified in :pep:`503`
+and updated over subsequent PEPs.
+
+In this context of this PEP, "freezing" means that the HTML representation
+is considered complete from the perspective of the standards process,
+and **SHOULD NOT** be updated by future PEPs. Future PEPs **SHOULD** instead
+target the
+:ref:`standard JSON representation <packaging:simple-repository-api-json>`,
+as originally specified in :pep:`691`.
+
+Similarly, this PEP's freezing of the HTML representation does **not** stipulate
+that installers should remove support for the HTML representation, or that
+indices (like PyPI) will or should stop providing an HTML representation.
+
+Rationale and Motivation
+========================
+
+The use of an HTML representation for Python package indices predates
+efforts to standardize Python packaging. Consequently, the HTML representation
+standardized with :pep:`503` represents a *formalization* of
+existing practices (particularly those of PyPI), rather than a *design*.
+
+The HTML representation of a Python package index has served the Python
+packaging ecosystem admirably: it has acted as the baseline representation
+that all indices and installers support, and has allowed PyPI to incrementally
+modernize its index presentation while maintaining backwards compatibility
+with installers and mirrors. :pep:`629`, :pep:`714`, :pep:`740`,
+:pep:`792`, and many others demonstrate the viability of this approach.
+
+At the same time, the HTML representation has several limitations that
+have become increasingly apparent and salient as Python packaging as a whole
+has modernized:
+
+- The HTML representation is *rigid*, for backwards compatibility reasons.
+  This rigidity makes it difficult to represent new pieces of metadata,
+  and PEPs that attempt to do so typically need to shoehorn their changes
+  into ``<meta>`` tags or ``data-`` attributes to avoid interfering with
+  assumptions that existing consumers make about the structure of the HTML.
+
+  This shoehorning process also requires PEPs that modify the HTML index
+  to invent syntax for encoding structured data. For example, :pep:`792`
+  adds meta tags named ``pypi:project-status`` and
+  ``pypi:project-status-reason``, effectively flattening an object
+  representation that appears naturally in the JSON representation.
+
+  Similarly, the HTML representation's rigidity makes it an optimization
+  barrier: :pep:`658` allows indices to serve distribution metadata via
+  the simple repository API, but the absence of a straightforward and
+  backwards-compatible way to encode that metadata within the HTML
+  representation means that installers must incur an additional HTTP round-trip
+  to fetch relatively small amounts of information. :pep:`740` adopts a
+  similar approach, with similar overhead repercussions.
+
+  In practice, some index PEPs have chosen not to modify the HTML representation
+  at all, and instead focus solely on the JSON representation. :pep:`700`
+  for example introduces both per-distribution metadata *and* a top-level
+  ``versions`` key to the JSON representation, but does not modify the HTML
+  representation. The original rationale for this was that HTML consumers
+  would be unlikely to need the new metadata,
+
+- Relatedly, third-party consumption of the HTML representation is often
+  *brittle*: even syntactically valid, non-semantic changes to PyPI's HTML
+  representation are
+  `known to cause breakage <https://github.com/pypi/warehouse/issues/18275>`__
+  due to unsound assumptions about the exact structure of the HTML, including
+  its whitespace.
+
+  Consumption of the JSON representation, by contrast, is more robust to
+  non-semantic changes thanks to the prevalence of robust JSON parsing
+  libraries. Robust handling of HTML is naturally possible, but consumers
+  are often *tempted* to avoid the perceived complexity and generality
+  of HTML parsing in favor of brittle approaches involving regular expressions
+  and similar ad-hoc parsing techniques.
+
+- In practice, *adoption* of incremental improvements to the HTML representation
+  is limited: PyPI itself typically adopts new features, but third-party
+  indices (particularly those sold as corporate offerings) frequently provide
+  only the absolute minimum representation originally defined in :pep:`503`.
+
+  As a result, *even when* the HTML representation is improved, many consumers
+  do not benefit from those improvements.
+
+Put together, these limitations mean that the HTML representation is (1)
+often difficult to extend in a robust way, (2) *de facto* frozen with
+respect to how many consumers interact with Python packaging, even
+when standards processes work to modernize it.
+
+The purpose of this PEP is to formalize this status quo.
+
+Specification
+=============
+
+The HTML representation of the simple repository API is frozen
+for the purposes of Python packaging standards processes. Future
+Python packaging PEPs **SHOULD NOT** modify the HTML representation of the
+simple repository API, and **MUST** instead modify the JSON representation.
+
+This PEP does not alter the status of the HTML representation on PyPI
+and does not prescribe any behavioral changes for installers.
+
+One functional consequence of this freeze is that future changes
+to the simple repository API will be
+:ref:`versioned <packaging:simple-repository-api-versioning>` as they are
+currently, but that only the JSON representation will receive changes
+to its versioning marker. For example, if a future PEP introduces
+version 1.5 of the simple repository API, the HTML representation will retain
+the following versioning marker:
+
+.. code-block:: html
+
+   <meta name="pypi:repository-version" content="1.4">
+
+Future Considerations
+=====================
+
+This PEP does not stipulate any changes to how indices and installers should
+handle the HTML representation.
+
+As of April 2026, the prospect of *fully* removing support for the HTML
+representation from either indices or installers is unrealistic: it is simply
+too critical to the ecosystem, and efforts to remove it would be extremely
+and unreasonably disruptive.
+
+However, it is not *inconceivable* that the HTML representation could be
+fully removed (or relegated to legacy/default-disabled flows) in the future.
+This PEP does not preclude such a future, but does not propose it either.
+
+The Python packaging community has made several valuable observations
+around behaviors that make outright removal of the HTML representation
+difficult or infeasible, including:
+
+- By virtue of being the default, the HTML representation is extremely
+  easy to adopt internally: it doesn't require any (explicit) content
+  negotiation, and can often be served trivially by a CDN or a minimal
+  HTTP server (like ``python -m http.server``).
+
+  The JSON representation does not technically require content negotiation
+  either, but in practice clients that consume it expect to perform
+  explicit content negotiation due to the assumption that the same URL
+  provides both representations. Consequently, any future efforts to remove the
+  HTML representation will likely require a simpler adoption story for the JSON
+  representation.
+
+- The HTML representation is currently easier for installers like ``pip``
+  to parse incrementally, as the Python standard library includes
+  ``html.parser`` for incremental HTML parsing. This helps mitigate
+  the memory overhead of large HTML index responses, e.g. detail responses
+  for packages that have hundreds or thousands of distributions.
+
+  By contrast, Python's standard library currently lacks an incremental
+  JSON parser. Incremental JSON parsing is not impractical (and is strictly
+  less complex than incremental HTML parsing), but the absence of a
+  standard library solution presents an adoption barrier.
+  Future efforts to remove the HTML representation will likely require a robust
+  standard library (or acceptably vendorable third-party) solution for
+  incremental JSON parsing within ``pip``.
+
+Security Implications
+=====================
+
+This PEP does not identify and positive or negative security implications
+associated with freezing the HTML representation of the simple repository
+API.
+
+How to Teach This
+=================
+
+Because this PEP only freezes the HTML representation of the simple repository
+API for the purposes of Python packaging standards processes, the end user
+implications of this PEP are limited.
+
+However, for third-party indices that wish to modernize their index
+representations, this PEP proposes the following if accepted:
+
+- The authors of this PEP will coordinate with the maintainers
+  of PyPI on appropriate public-facing documentation and communication,
+  including an announcement on the `PyPI blog <https://blog.pypi.org>`_
+  if deemed appropriate.
+
+- The authors of this PEP will make appropriate changes to the
+  :ref:`living standard <packaging:simple-repository-api>` for the simple
+  repository API, including admonitions and callouts where appropriate
+  to indicate that the HTML representation will not receive future updates.
+
+Rejected Ideas
+==============
+
+Doing nothing
+-------------
+
+Doing nothing is always an option. Per above, this would be a continuation
+of the status quo, wherein the HTML representation is updated on paper
+(and on PyPI), but is frozen in practice in third-party settings.
+
+The authors of this PEP believe that being explicit about the status
+of the HTML representation is valuable, and would benefit future standards
+efforts by diverting design effort away from shoehorning new features
+into the HTML representation.
+
+
+Aggressively removing the HTML representation
+---------------------------------------------
+
+Encouraging indices and installers to aggressively remove support for the HTML
+representation is another option. However, as noted above, this is unrealistic
+in the near term, and would be disruptive to the ecosystem.
+
+The authors of this PEP believe that freezing is a more gradual and
+pragmatic approach that better reflects the ecosystem's reality.
+
+Copyright
+=========
+
+This document is placed in the public domain or under the CC0-1.0-Universal
+license, whichever is more permissive.