Define the supported HTML subset for message events

Also clarify that `m.notice` messages can support HTML.

Fixes https://github.com/matrix-org/matrix-doc/issues/1559
Fixes https://github.com/matrix-org/matrix-doc/issues/1560

event-schemas/examples/m.room.message#m.notice

@@ -2,7 +2,9 @@
   "age": 242352,
   "content": {
     "body": "This is an example notice",
-    "msgtype": "m.notice"
+    "msgtype": "m.notice",
+    "format": "org.matrix.custom.html",
+    "formatted_body": "This is an <strong>example</strong> notice"
   },
   "origin_server_ts": 1431961217939,
   "event_id": "$WLGTSEFSEF:localhost",

specification/modules/instant_messaging.rst

@@ -56,6 +56,54 @@ of message being sent. Each type has their own required and optional keys, as
 outlined below. If a client cannot display the given ``msgtype`` then it SHOULD
 display the fallback plain text ``body`` key instead.
 
+Some message types support HTML in the event content that clients should prefer
+to display if available. Currently ``m.text``, ``m.emote``, and ``m.notice``
+support an additional ``format`` parameter of ``org.matrix.custom.html``. When
+this field is present, a ``formatted_body`` with the HTML must be provided. The
+plain text version of the HTML should be provided in the ``body``.
+
+Clients should limit the HTML they render to avoid Cross-Site Scripting, HTML 
+injection, and similar attacks. The strongly suggested set of HTML tags to permit,
+denying the use and rendering of anything else, is: ``font``, ``del``, ``h1``, 
+``h2``, ``h3``, ``h4``, ``h5``, ``h6``, ``blockquote``, ``p``, ``a``, ``ul``,
+``ol``, ``sup``, ``sub``, ``nl``, ``li``, ``b``, ``i``, ``u``, ``strong``, ``em``,
+``strike``, ``code``, ``hr``, ``br``, ``div``, ``table``, ``thead``, ``tbody``,
+``tr``, ``th``, ``td``, ``caption``, ``pre``, ``span``, ``img``.
+
+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 listed for the tags below.
+Where ``data-mx-bg-color`` and ``data-mx-color`` are listed, clients should 
+translate the value (a 6-character hex color code) to the appropriate CSS/attributes
+for the tag.
+
+
+:``font``:
+  ``data-mx-bg-color``, ``data-mx-color``
+
+:``span``:
+  ``data-mx-bg-color``, ``data-mx-color``
+
+:``a``:
+  ``name``, ``target``, ``href`` (provided the value is not relative and has a scheme
+  matching one of: ``https``, ``http``, ``ftp``, ``mailto``, ``magnet``)
+
+:``img``:
+  ``width``, ``height``, ``alt``, ``title``, ``src`` (provided it is a Matrix Content
+  URI)
+
+:``ol``:
+  ``start``
+
+:``code``:
+  ``class`` (only classes which start with ``language-`` for syntax highlighting)
+
+
+Additionally, clients should ensure that *all* ``a`` tags get a ``rel="noopener"``
+to prevent the target page from referencing the client's tab/window.
+
+
+
 {{msgtype_events}}