R Markdown doesn't use LaTeX when producing HTML. The code you used would work if output was to pdf_document
, but not to html_document
.
If you really want the labelled list in HTML, you're going to have to insert HTML code, not LaTeX code. I don't know if there's anything visually equivalent to LaTeX's \item[R=]
, but you could do this, which is logically equivalent:
---
title: "Untitled"
author: "March 2019"
output: html_document
---
```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = TRUE)
```
\[x = R + E\]
where:
<dl>
<dt>R=</dt>
<dd>is Racoon</dd>
<dt>E=</dt>
<dd>is Elephant</dd>
</dl>
This displays as

Perhaps CSS could be crafted to make it visually equivalent.
Edited to add: And of course it is possible, and has been done. It's easy to follow https://stackoverflow.com/a/13371522/2554330, since R Markdown uses the Bootstrap framework. Just add a class to the first tag:
<dl class="dl-horizontal">
<dt>R=</dt>
<dd>is Racoon</dd>
<dt>E=</dt>
<dd>is Elephant</dd>
</dl>
and it produces

If the styling is still not satisfactory, look at some of the other discussion related to https://stackoverflow.com/a/13371522/2554330.