Deprecate the font HTML tag (#1739)

* Deprecate the `font` HTML tag

Rationale:
MSC4077 allows to deprecate HTML tags
that are deprecated in the WHATWG standard,
if they can be replaced by tags with the same feature.

`font` is deprecated and can be replaced by `span`
with the `data-mx-bg-color` and `data-mx-color` attributes.

Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>

* Add changelog

Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>

* Add deprecation info box

Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>

---------

Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>

changelogs/client_server/newsfragments/1739.clarification

@@ -0,0 +1 @@
+The [font](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/font) element is deprecated in the HTML spec. Clients should prefer [span](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/span) with the `data-mx-bg-color` and `data-mx-color` attributes instead.

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

@@ -43,10 +43,10 @@ were limited to `m.text`, `m.emote`, `m.notice`, and
 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`, `li`, `b`, `i`, `u`, `strong`, `em`,
-`s`, `code`, `hr`, `br`, `div`, `table`, `thead`, `tbody`, `tr`,
-`th`, `td`, `caption`, `pre`, `span`, `img`, `details`, `summary`.
+`del`, `h1`, `h2`, `h3`, `h4`, `h5`, `h6`, `blockquote`, `p`, `a`, `ul`,
+`ol`, `sup`, `sub`, `li`, `b`, `i`, `u`, `strong`, `em`, `s`, `code`,
+`hr`, `br`, `div`, `table`, `thead`, `tbody`, `tr`, `th`, `td`,
+`caption`, `pre`, `span`, `img`, `details`, `summary`.
 
 {{% boxes/note %}}
 {{% added-in v="1.10" %}}
@@ -55,6 +55,14 @@ requiring a [Spec Change Proposal](/proposals) when they are deprecated in the
 [WHATWG HTML Living Standard](https://html.spec.whatwg.org/multipage/).
 {{% /boxes/note %}}
 
+{{% boxes/note %}}
+{{% changed-in v="1.10" %}}
+In previous versions of the specification, the `font` tag was suggested with the
+`data-mx-bg-color`, `data-mx-color` and `color` attributes. This tag is now
+deprecated in favor of the `span` tag with the `data-mx-bg-color` and
+`data-mx-color` attributes in new messages.
+{{% /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
@@ -65,7 +73,6 @@ the tag.
 
 | Tag    | Permitted Attributes                                                                                                                       |
 |--------|--------------------------------------------------------------------------------------------------------------------------------------------|
-| `font` | `data-mx-bg-color`, `data-mx-color`, `color`                                                                                               |
 | `span` | `data-mx-bg-color`, `data-mx-color`, `data-mx-spoiler` (see [spoiler messages](#spoiler-messages))                                         |
 | `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 (`mxc://`) URI](#matrix-content-mxc-uris))                      |