Using command line I can do 
````shell
pandoc --from markdown+emoji --to revealjs -s -o test.html test.qmd
````

Is this supporting in quarto ? I tried a few thing with odd result

Doing 
```
---
format: revealjs
variant: +emoji
---
````

will lead to  adding the extension to the `to` format

````
pandoc 
  to: revealjs+emoji
  from: markdown
  standalone: true
  filters:
    - crossref
 ````

I also tried at command line but no luck passing the `--from` argument from pandoc command line to `quarto render`. It does the same (moving the extensions to the to format)

```
❯ quarto render test2.qmd --from markdown+emoji
pandoc --from markdown+emoji
  to: revealjs+emoji
  from: markdown
  standalone: true
  filters:
    - crossref

metadata
  hash: true

The extension emoji is not supported for revealjs
```

````
❯ quarto render test2.qmd --to revealjs --from markdown+emoji
pandoc --from markdown+emoji
  to: revealjs+emoji
  from: markdown
  standalone: true
  filters:
    - crossref

metadata
  hash: true

The extension emoji is not supported for revealjs
````


## test file 

````markdown
---
title: test
format: revealjs
# variant: +emoji
---

# Slide with emoji

Here is a emoji

:smile:
````


2021-07-23T16:39:02Z
cderv
closed
Passing non default extension to markdown input format 
https://github.com/quarto-dev/quarto-cli/issues/60