MSC2010: Add client-side spoilers

proposals/2010-spoilers.md

@@ -0,0 +1,67 @@
+# MSC 2010: Proposal to add client-side spoilers
+Sometimes, while you want to put text into a spoiler to not have people accidentally read things that they don't want to see.
+
+For example, when discussing a new movie or a TV series, not everyone might have watched it yet.
+In such cases it would make sense to add a spoiler so that only those who have seen the movie or
+don't mind spoilers read the content.  
+Another example would be e.g. in mental health communities where certain people have certain
+triggers. People could put talking about abuse or thelike into a spoiler, to not accidentally
+trigger anyone just reading along the conversation.  
+Furthermore this is helpful for bridging to other networks that already have a spoiler feature.
+
+To render the spoiler the content is hidden and then revealed once interacted somehow
+(e.g. a click / hover).
+
+## Proposal
+This proposal is about adding a new attribute to the `formatted_body` of messages with type
+`m.room.message` and msgtype `m.text`.
+
+It adds a new attribute, `data-mx-spoiler`, to the `<span>` tag. If the attribute is present the
+contents of the span tag should be rendered as a spoiler. Optionally, you can specify a reason for
+the spoiler by setting the attribute string. It could be rendered, for example, similar to this:
+
+![Spoiler rendering idea](https://user-images.githubusercontent.com/2433620/59299700-95063480-8c8d-11e9-9348-3e2c8bc94bdc.gif)
+
+The plain-text fallback could be rendered as `(Spoiler: <content>)` and
+`(Spoiler for <reason>: <content>)` respectively.
+
+### Example
+Without reason:
+```
+{
+    "msgtype": "m.text",
+    "format": "org.matrix.custom.html",
+    "body": "Hello there, the movie was (Spoiler: awesome)",
+    "formatted_body": "Hello there, the movie was <span data-mx-spoiler>awesome</span>"
+}
+```
+With reason:
+```
+{
+    "msgtype": "m.text",
+    "format": "org.matrix.custom.html",
+    "body": "Hey (Spoiler for movie: the movie was awesome)",
+    "formatted_body": "Hey <span data-mx-spoiler="movie">the movie was awesome</span>"
+}
+```
+
+## Tradeoffs
+Instead of making this an attribute, an entirely new tag could be introduced (e.g. `<mx-spoiler>`),
+however that wouldn't be HTML-compliant.
+
+Instead of limiting the proposed `data-mx-spoiler` attribute only to the `<span>`-tag it could be
+added to all tags, however it might make implementations for clients more complicated.
+
+Clients would have to come up with a way how to input spoilers. This could be done, for example,
+by adding a custom markdown tag (like discord does), so that you do `Text ||spoiler||`, however
+that doesn't take a spoiler reason into account.
+
+## Potential issues
+Depending on context it might make sense to put other events, such as `m.image`, into spoilers,
+too. This MSC doesn't address that at all. Using
+`<span data-mx-spoiler><img src="mxc://server/media"></span>` seems rather sub-optimal for that.
+
+This MSC doesn't take HTML block elements into account.
+
+## Security considerations
+The spoiler reason needs to be properly escaped when rendered.