Doesn't work with RevealJS HTML Backend

## I am using it as follows

#+begin_parallel
[[color:orange][Are you excited to learn some Lisp?]] [[blue:Yes!]]

Pop-quiz: How does doc:apply work?
#+end_parallel

## In the HMTL backend,

it works fine.

## However, when I export it to revealJS HTML

Using the `org-reveal` package, the generated HTML doesn't reflect the different columns as it does in plain HTML. As I inspect the generated HTML using the reveal backend, it has no special CSS inserted as it had in the plain HTML backend.

## Please tick the following, by replacing `[ ]` with `[X]`

1. [x] I am aware of the extensive documentation at http://alhassy.com/org-special-block-extras/
2. [ ] I have read the documentation of doc:org-defblock
3. [ ] I have checked some boxes without reading them.
4. [ ] I am aware that `(org-defblock X ⋯)` gives me a Lisp function `org-block/X`
that I can play with; e.g., `C-h o org-block/shout` shows a function with its
docstring and arguments. We can press `C-u C-x C-e` at the end of the
closing parens of `(org-block/shout 'html "*Hello*, /world/!")` to see
what _Org sees_ when it rewrites a `shout` block with the given string as its contents.