Allow the use of the HTML <details> tag

Signed-off-by: Alexander Olofsson <ace@haxalot.com>

proposals/2184-allow-html-details.md

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+# Allow the HTML `<details>` tag in messages
+
+Currently, there's no available method for bot developers - among others - to provide larger informative
+messages in a room without disrupting the conversation for all other users. This often causes bots
+to appear needlessly "spammy".
+
+## Proposal
+
+This proposal suggests adding the existing HTML tags for `<details>` and `<summary>` to the list of
+allowed tags in formatted Matrix messages. Which would allow for larger messages to be shown simply
+as smaller - and less intrusive - summaries for the users who are not interested in their full contents.
+
+## Tradeoffs
+
+An alternative method to provide a summary/details split could possibly be done through [MSC1767],
+with the details and summaries being specified through repeated bodies with added metadata. This could
+then also allow clients better autonomy in deciding what to display - or how to structure the information.
+
+However, allowing the use of the `<details>` and `<summary>` tags would still offer richer formatting
+capabilities even in such messages, especially as more than one detail/summary block could be included
+in a single message.
+
+## Potential issues
+
+Allowing more HTML tags in formatted messages could cause more work for client developers, as they would
+have to fit a larger and more diverse corpus of input into their designs and user experience.
+However, these are both well documented - and implemented - HTML tags, so there is plenty of prior work
+available to take example from in how to incorporate them.
+
+Additionally, as the addition of these tags will make it possible to fit even more information into
+a single message without worry of overflowing the room, any client that doesn't render the formatting
+of the body might end up with a lessened user experience - from either an under- or overflow of information.
+
+The onus on ensuring that the unformatted body is a reasonable representation of the message has always
+been on the user or bot writing the formatted message though, so providing an improved ability for
+formatting should not negatively affect the experience for any clients that simply render unformatted
+text.
+
+## Security considerations
+
+Allowing more HTML tags in client rendering could lead to a wider attack surface for DOM-based exploits.
+However, these tags are very simple in both function and design, so any possible attack surface they
+would offer would be minimal at best.
+
+[MSC1767]: https://github.com/matrix-org/matrix-doc/blob/matthew/msc1767/proposals/1767-extensible-events.md