Experimenting with Canvas yesterday, I discovered that the Rich Content Editor no longer strips out two HTML5 tags that allow you to easily add captions to images: <figure> and <figcaption>
This could be very useful when, for example, adding a copyright notice to an image. However, these tags are not on the Canvas HTML whitelist. Does that mean it's a mistake and the tags may be stripped out in the future? I reached out to Canvas support, and this is what they said:
There can be other HTML tags out there that won't be stripped out, <figure> and <figcaption> just being a few. Since they're not on the whitelist, I can't guarantee they won't start being stripped out at some point in the future, but at the same time, it's not something we're going to actively disallow just because it's not whitelisted. So in short, you're free to continue using those tags keeping in mind it could be disallowed in the future.
Now, if it were up to me I would greatly expand the whitelist to include a range of HTML5 and CSS3 tags (drop shadows, rounded corners, etc.). But maybe this is something they'll address when they redesign the RCE.
I may not be 100% on this, but it looks like <mark> tag for highlighting text is also available to use. I searched the allowed HTML and the only place it came up was under MathML.