Add a CSS class to single code chunks in RMarkdown - css

Question

Is it possible to add a CSS class to a certain code chunk?
Assume the following file:
---
title: "Untitled"
output: html_document
---
```{r cars}
summary(cars)
```
I want to give the chunk labeled 'cars' a certain CSS class, e.g. .myClass.
Is there any possibility like
```{r cars} {.myClass}
summary(cars)
```
or so? I am aware of hacks like wrapping the whole chunk in another <div>. I am interested in a straight forward solution.

Answer 1

Edit: this feature was introduced in knitr v.1.16 (05/18/17)
class.source and class.output options apply additional HTML classes to source and output chunks (see knitr documentation).
To add myClass to source chunk:
```{r cars, class.source='myClass'}
summary(cars)
```
Previous answer that inspired the class.source options (see here)
You can add a class using the fenced_code_attributes pandoc's extension (which is intended to add attributes to the <pre> tag, see here) and a knitr output hook.
The following example works fine:
---
title: "Untitled"
output:
html_document:
md_extensions: +fenced_code_attributes
---
```{r, include=FALSE}
knitr::knit_hooks$set(source = function(x, options) {
return(paste0(
"```{.r",
ifelse(is.null(options$class),
"",
paste0(" .", gsub(" ", " .", options$class))
),
"}\n",
x,
"\n```"
))
})
```
```{r cars, class="myClass1 myClass2"}
summary(cars)
```
After knitting this .Rmd file, the HTML document looks like this:
<pre class="r myClass1 myClass2">
<code>
summary(cars)
</code>
</pre>
The fenced_code_attributes extension is enabled by default: in standard cases, you don't need to include the line md_extensions: +fenced_code_attributes in your YAML header.
I don't know if there's more straightforward solution using knitr.