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+# MSC4003: Semantic table attributes
+
+The specification currently [advises][msgtypes] clients to malform HTML tables
+by removing attributes that encode table data. For example, if a message
+contains the following table—
+
+>
+>
+> | A | B | C |
|---|
+>
+>
+> | D | E |
+>
+>
+
+—the specification advises that it be transformed to:
+
+>
+>
+> | A | B | C |
|---|
+>
+>
+> | D | E |
+>
+>
+
+To maintain message integrity, HTML attributes that encode table _data_ should
+be permitted. These attributes pose no security risk, serve a narrowly defined
+semantic role, and are a natural completion of the existing permissions.
+
+## Proposal
+
+### Process
+
+To identify which attributes should be permitted, apply the following
+constraints:
+
+1. Constrain to the [permitted tags][msgtypes] of the existing specification.
+
+ > `table`, `thead`, `tbody`, `tr`, `th`, `td`
+
+ Rationale: The purpose of this MSC is to correct a technical oversight in
+ the existing specification while preserving its intent, not to add new
+ functionality.
+
+1. Constrain to the [permitted attributes][Sanitize] of a typical sanitizer.
+
+ > | Tag | Permitted attributes |
+ > | ------- | ------------------------------------------------------------------------------------------------------------ |
+ > | `table` | `align`, `bgcolor`, `border`, `cellpadding`, `cellspacing`, `frame`, `rules`, `sortable`, `summary`, `width` |
+ > | `th` | `abbr`, `align`, `axis`, `colspan`, `headers`, `rowspan`, `scope`, `sorted`, `valign`, `width` |
+ > | `td` | `abbr`, `align`, `axis`, `colspan`, `headers`, `rowspan`, `valign`, `width` |
+
+ Rationale: Security takes priority over other concerns.
+
+1. Constrain to attributes that encode content (not
+ [presentation][separation of content and presentation]):
+
+ > | Tag | Permitted attributes |
+ > | ------- | ------------------------------------------------------------------------------------------------------------------------------------------------ |
+ > | `table` | ~~`align`~~, ~~`bgcolor`~~, ~~`border`~~, ~~`cellpadding`~~, ~~`cellspacing`~~, ~~`frame`~~, ~~`rules`~~, ~~`sortable`~~, `summary`, ~~`width`~~ |
+ > | `th` | `abbr`, ~~`align`~~, `axis`, `colspan`, `headers`, `rowspan`, `scope`, ~~`sorted`~~, ~~`valign`~~, ~~`width`~~ |
+ > | `td` | `abbr`, ~~`align`~~, `axis`, `colspan`, `headers`, `rowspan`, ~~`valign`~~, ~~`width`~~ |
+
+ Rationale: The scope of this MSC is data integrity. Presentation is the
+ responsibility of the client.
+
+1. Constrain to [standard] attributes:
+
+ > | Tag | Permitted attributes |
+ > | ------- | ------------------------------------------------------------ |
+ > | `table` | ~~`summary`~~ |
+ > | `th` | `abbr`, ~~`axis`~~, `colspan`, `headers`, `rowspan`, `scope` |
+ > | `td` | ~~`abbr`~~, ~~`axis`~~, `colspan`, `headers`, `rowspan`, |
+
+ Rationale: Strict adherence to open standards supports interoperability and
+ accessibility.
+
+### Results
+
+Following the above process, the resulting proposed permissions are:
+
+| Tag | Permitted attributes |
+| ---- | ---------------------------------------------------------- |
+| `th` | [`abbr`], [`colspan`], [`headers`], [`rowspan`], [`scope`] |
+| `td` | [`colspan`], [`headers`], [`rowspan`] |
+
+## Potential issues
+
+In practice the `headers` attribute is useless without corresponding `id`
+attributes, but robust handling of `id` attributes in user-generated content is
+a broader topic with potentially complex needs and should be discussed
+separately. For the purposes of this MSC it would suffice to simply omit the
+`headers` attribute, but including it here has negligible cost and the benefit
+of separating concerns in the long term.
+
+## Security considerations
+
+As specified, the `abbr`, `colspan`, `headers`, `rowspan`, and `scope`
+attributes have no potential for causing effects outside of the table. All are
+permitted in default configurations of popular HTML sanitizers:
+
+- [Sanitize] (used by GitHub)
+- [Loofah] (used by Rails)
+- [Bleach] (popular Python library)
+
+Exceptions:
+
+- [sanitize-html] (used by Element) doesn’t include them, but is a younger
+ project with currently incoherent defaults that are unfit as a reference for
+ any use case.
+- [Ammonia] (popular Rust library) permits most of the proposed attributes but
+ omits `abbr` without explanation. From skimming its version history its
+ defaults appear to have been formed through ad hoc feedback rather than
+ principled research.
+
+[`abbr`]: https://html.spec.whatwg.org/multipage/tables.html#the-th-element
+[Ammonia]: https://github.com/rust-ammonia/ammonia/blob/v3.3.0/src/lib.rs#L422-L442
+[Bleach]: https://github.com/mozilla/bleach/blob/v6.0.0/bleach/_vendor/html5lib/filters/sanitizer.py#L195
+[`colspan`]: https://html.spec.whatwg.org/multipage/tables.html#attributes-common-to-td-and-th-elements
+[`headers`]: https://html.spec.whatwg.org/multipage/tables.html#attributes-common-to-td-and-th-elements
+[Loofah]: https://github.com/flavorjones/loofah/blob/v2.20.0/lib/loofah/html5/safelist.rb#L231
+[msgtypes]: https://spec.matrix.org/v1.6/client-server-api/#mroommessage-msgtypes
+[`rowspan`]: https://html.spec.whatwg.org/multipage/tables.html#attributes-common-to-td-and-th-elements
+[Sanitize]: https://github.com/rgrove/sanitize/blob/v6.0.1/lib/sanitize/config/relaxed.rb#L27-L29
+[sanitize-html]: https://github.com/apostrophecms/sanitize-html/blob/v1.9.0/index.js#L295
+[`scope`]: https://html.spec.whatwg.org/multipage/tables.html#the-th-element
+[separation of content and presentation]: https://en.wikipedia.org/wiki/Separation_of_content_and_presentation
+[standard]: https://html.spec.whatwg.org/multipage/tables.html