HTML rb, rtc and rbc elements are deprecated

[palemieux]

See https://developer.mozilla.org/en-US/docs/Web/HTML/Element/rtc and https://developer.mozilla.org/en-US/docs/Web/HTML/Element/rb

rbc might have never been officially supported (whatwg/html#101).

rtc is currently used at

imscJS/src/main/js/html.js

Line 216 in a6ed5f4

e = document.createElement("rtc");

rbc is currently used at

imscJS/src/main/js/html.js

Line 211 in a6ed5f4

e = document.createElement("rbc");

[palemieux]

It looks like there is a discussion to add some of the functionality back:

whatwg/html#1771

[nigelmegitt]

We did also have rb which is also marked as deprecated in the WHATWG HTML spec; it just got removed in #238.