R Markdown document with html/docx output, using LaTeX package bbm?

Question

How can I use the LaTeX package bbm in an R markdown for html/docx output?

Currently I'm using the hack solution below where, essentially, I just abandon using the package bbm for .docx/.html output. Is there a hack solution in which I can still use the package?

Note, this question related to my question In an R Markdown document, “includes” for docx output? where there I'm specifically asking about how to move these special <!--- For DOCX Only ---> code chunk to a preamble-word.tex file to be indcluded in the YAML header. This question is also related to the question How to get \bm{} to work in an R markdown (to HTML) file? My current hack for bbm is basically an adaptation of one of the hacks proposed as an answer to that question.

tinytextest.Rmd

---
title: "TinyTeX Test"
header-includes:
  - \usepackage{bbm}
output:
  pdf_document: default
  html_document: default
  word_document: default
---

```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = TRUE)
```

<!--- For HTML Only --->
`r if (knitr:::is_html_output()) '
$\\newcommand{\\mathbbm}[1]{\\mathbf{#1}}$
'`

<!--- For DOCX Only --->
`r if (!knitr:::is_latex_output() & !knitr:::is_html_output()) '
\\newcommand{\\mathbbm}[1]{\\mathbf{#1}}
'`

Hello

\[\mathbbm{1}_{S}(x)\]

.pdf output

.docx/.html output

Answer 1

Ok, another answer, I just realized, these are two slightly different questions - one by the question starter and one by the bounty starter.

I would need a solution to this issue as well. Here is my minimum example expected to work:

---
title: "Testing LaTeX packages in Docx format" 
output: word_document 
header-includes:
  - \usepackage{xfrac} 
--- 
  $$ \sfrac{1}{2} $$ ```

In this case the outlined answer with MathJax will not work so well. sfrac is not supported by MathJax (http://docs.mathjax.org/en/latest/input/tex/macros/index.html)

I don't know if you need \sfrac specifically, but \frac or \tfrac would be supported. Maybe you can use these instead.

Here an example with \frac

 ---
 title: "Frac Test"
 header-includes: 
  - \usepackage{xfrac}
 output:
  pdf_document: default
  html_document: default
  word_document: default
---

$$
   \frac{1}{2}+1
$$

Output of \frac example html:

This will work for .pdf / word and html.

Here an example with \tfrac

---
title: "TFrac Test"
header-includes: 
 - \usepackage{xfrac}
output:
  pdf_document: default
  html_document: default
  word_document: default
---

$$
   \tfrac{1}{2}+1
$$

Output \tfrac example html:

This will run for html, pdf - for word it will write 1/2 instead of a formula, since there is no \tfrac equivalent in word.

\cfrac and \dfrac are also supported by MathJax.