MSC2422: Allow color on font tag

Signed-off-by: Nicolas Werner <nicolas.werner@hotmail.de>

proposals/2422-allow-color-attribute-on-font-tag.md

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+# MSC2422: Allow `color` as attribute for `<font>` in messages
+
+Currently the spec requires you to use `data-mx-color` instead of the standard
+`color` html attribute for the `<font>` tag. This is probably done to make it
+consistent with `<span>`, where you may not want to allow a generic style tag for.
+
+On the other hand the /rainbow command on almost every client just uses the
+`color` attribute of the `<font>` tag. While some clients support
+`data-mx-color` (i.e. Riot Web), most clients don't. Most clients support
+rendering `color` however.
+
+It would probably be for the best to allow or even prefer `color` on the
+`<font>` tag.
+
+## Proposal
+
+Add the `color` attribute to the allowed attributes of `<font>` in section
+13.2.1.7. Allow only hexcodes as allowed value for this attribute for now.
+
+## Potential issues
+
+- We now have a redundant attribute in the spec. While it matches, what the
+    clients currently do, it may be better to fix each client instead.
+- Clients may not sanitize the color attribute and will let other color values
+    through, increasing compatibility issues again.
+- Clients may never support the data-mx-* attributes now.
+- Old messages could loose their color
+- This proposal doesn't touch span at all, maybe it should?
+
+## Alternatives
+
+- fix the clients
+- remove the `data-mx-color` and `data-mx-bg-color` attributes entirely, leaving
+    us just with `color` for `<font>`
+- Add a section to tell the clients to prefer `color` over `mx-data-color`
+- Spec an entirely different format for messages (that would probably not make
+    this proposal obsolete)
+