Document the deprecation policy of HTML tags. (#1732)

* Deprecate `strike` HTML tag

Replace it with `s` or `del`.

Signed-off-by: Kévin Commaille <[email protected]>

* Add changelog

Signed-off-by: Kévin Commaille <[email protected]>

* Remove strike deprecation and add added-in annotation

Signed-off-by: Kévin Commaille <[email protected]>

* Add link to HTML standard

Co-authored-by: Hubert Chathi <[email protected]>

* Update changelogs/client_server/newsfragments/1732.clarification

---------

Signed-off-by: Kévin Commaille <[email protected]>
Co-authored-by: Hubert Chathi <[email protected]>
Co-authored-by: Travis Ralston <[email protected]>

changelogs/client_server/newsfragments/1732.clarification

@@ -0,0 +1 @@
+Document the deprecation policy of HTML tags, as per [MSC4077](https://github.com/matrix-org/matrix-spec-proposals/pull/4077).

content/client-server-api/modules/instant_messaging.md

@@ -48,6 +48,13 @@ tags to permit, denying the use and rendering of anything else, is:
 `s`, `code`, `hr`, `br`, `div`, `table`, `thead`, `tbody`, `tr`,
 `th`, `td`, `caption`, `pre`, `span`, `img`, `details`, `summary`.
 
+{{% boxes/note %}}
+{{% added-in v="1.10" %}}
+HTML features MAY be deprecated and replaced by their modern equivalent without
+requiring a [Spec Change Proposal](/proposals) when they are deprecated in the
+[WHATWG HTML Living Standard](https://html.spec.whatwg.org/multipage/).
+{{% /boxes/note %}}
+
 Not all attributes on those tags should be permitted as they may be
 avenues for other disruption attempts, such as adding `onclick` handlers
 or excessively large text. Clients should only permit the attributes