Add accessibility considerations regarding real-time text

Based on discussions with APA Working Group during TPAC https://www.w3.org/2019/09/20-apa-minutes.html#item02 and following discussions

webrtc.html

@@ -12928,6 +12928,51 @@ <h2>Setting SDP from remote endpoints</h2>
       SDP.</p>
     </section>
   </section>
+  <section class="informative">
+    <h2>Accessibility Considerations</h2>
+    <p>The WebRTC 1.0 specification exposes an API to control protocols
+      (defined within the IETF) necessary to establish real-time
+      audio, video and data exchange.</p>
+
+    <p>Real-time Text, defined in [[RFC 4103]] on which people
+      who are deaf or hard of hearing (among others) rely on for their communication needs, utilizes T.140
+      encapsulated in RTP to enable the transition from TDD/TTY devices to
+      IP-based communications. Support for RFC 4103 is especially important as it is
+      being established as the new and more reliable mechanism for text
+      communications with <a href="https://www.ecfr.gov/cgi-bin/text-idx?SID=ada7cebd65433a9e05c35426a2bc76b8&mc=true&node=pt47.3.67&rgn=div5">Public Safety Access Points (PSAP)</a>.</p>
+
+    <p>Since Real-time Text requires the ability to send and receive data in near
+real time, it can be best supported via the WebRTC 1.0 data channel API. As
+defined by the IETF, the data channel protocol utilizes the SCTP/DTLS/UDP
+protocol stack, which supports both reliable and unreliable data channels. The
+IETF chose to standardize SCTP/DTLS/UDP over proposals for an RTP data channel
+which relied on SRTP key management and were focused on unreliable
+communications.</p>
+
+    <p>Since the IETF chose a different approach than the RTP data channel as part of
+the WebRTC suite of protocols, as of the time of this publication there is no
+standardized way for the WebRTC APIs to directly support Real-time Text as
+defined at IETF and implemented in U.S. (FCC) regulations.  The WebRTC working
+Group will evaluate whether the developing
+IETF protocols in this space warrant direct exposure in the browser APIs
+and is looking for input from the relevant user communities on this
+potential gap.</p>
+
+Within the <a href="https://tools.ietf.org/wg/mmusic/">IETF MMUSIC Working Group</a>, work is ongoing to enable <a href="https://tools.ietf.org/html/draft-holmberg-mmusic-t140-usage-data-channel">Real-time text to
+be sent over the WebRTC data channel</a>, allowing gateways to be deployed
+to translate between the SCTP data channel protocol and RFC 4103
+Real-time text. This work, once completed, would enable a unified and
+interoperable approach for integrating real-time text in WebRTC
+user-agents (including browsers) - through a gateway or otherwise.</p>
+
+<p>At the time of this publication, gateways that enable effective RTT
+support in WebRTC clients can be developed e.g. through a custom WebRTC
+data channel.  This is deemed sufficient until such time as future
+standardized gateways are enabled via IETF protocols such as the SCTP
+data channel protocol and RFC 4103 Real-time text. This will need to be
+defined at IETF in conjunction with related work at W3C groups to
+effectively and consistently standardise RTT support internationally.</p>
+  </section>
   <section class="appendix">
     <h2>Acknowledgements</h2>
     <p>The editors wish to thank the Working Group chairs and Team Contact,