MSC4077: Improved process for handling deprecated HTML features (#4077)

* MSC: Improved process for handling deprecated HTML features

* <3 spellcheck

* Apply suggestions from code review

Co-authored-by: David Baker <dbkr@users.noreply.github.com>
Co-authored-by: Hubert Chathi <hubert@uhoreg.ca>

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Co-authored-by: David Baker <dbkr@users.noreply.github.com>
Co-authored-by: Hubert Chathi <hubert@uhoreg.ca>

proposals/4077-process-deprecated-html.md

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+# MSC4077: Improved process for handling deprecated HTML features
+
+The Matrix specification [defines](https://spec.matrix.org/v1.8/client-server-api/#mroommessage-msgtypes)
+a subset of HTML which clients *should* use when rendering messages, limiting the possibility for
+Cross-Site Scripting (XSS) and similar attacks. Clients are always welcome to use a different subset
+in both their send and receive paths, however in practice the recommended set of HTML has become the
+baseline standard for support among Matrix client developers.
+
+Additionally, to fix issues like [tags being deprecated](https://github.com/matrix-org/matrix-spec/issues/1492),
+the existing spec process would require an entire MSC to add or remove the tag. This has precedent
+through [MSC2422](https://github.com/matrix-org/matrix-spec-proposals/pull/2422) and
+[MSC2184](https://github.com/matrix-org/matrix-spec-proposals/pull/2184), where additional HTML
+features are analyzed from a security perspective. For example, whether XSS attacks are more probable
+or flooding/disruption attempts are made easier.
+
+This proposal adjusts the MSC process to explicitly allow changes to the supported HTML tags where
+the WHATWG has deprecated a feature in favour of another, enabling more rapid iteration and reducing
+paperwork for everyone involved.
+
+## Proposal
+
+Where the WHATWG has deprecated a feature of the HTML specification, a Matrix spec PR *may* bypass the
+MSC process to use the modern feature instead. In the case of [issue#1492](https://github.com/matrix-org/matrix-spec/issues/1492),
+the spec PR is explicitly able to add the `s` tag and remove the `strike` tag. Note that such changes
+can also bypass the [deprecation policy](https://spec.matrix.org/v1.8/#deprecation-policy) in this way.
+
+It is left to the discretion of the spec PR reviewer as to whether an MSC is required. In general, an
+MSC should be requested when the replacement HTML feature is not quite the same as the old feature.
+The reviewer *may* also decide whether to keep supporting the old, deprecated, feature for some time.
+For example, replacing `strike` with `s` may mean that both tags are supported for 1-2 Matrix spec
+versions before `strike` can be dropped.
+
+For clarity, an MSC is always required to add net-new features to the HTML subset, or to remove a
+feature entirely. An example of this is [MSC2398](https://github.com/matrix-org/matrix-spec-proposals/pull/2398).
+These changes are subject to interoperability and security review, and cannot bypass the MSC process.
+
+## Potential issues
+
+The author notes that this MSC might not need to exist under the banner of pragmatism, however for all
+the effort to *not* write an MSC, an MSC has been written for review.
+
+## Alternatives
+
+Critically, without agreement that replacing deprecated HTML features can happen outside the MSC process,
+changes to the HTML subset become slow and burdensome for no apparent gain.
+
+## Security considerations
+
+Special care and attention is expected to be taken by spec PR reviewers to ensure that changes are
+safe and in line with the semantics of this proposal's guidance.
+
+## Unstable prefix
+
+Not applicable. This is a process MSC.
+
+## Dependencies
+
+None applicable.