r - Adding custom CSS tags to an RMarkdown html document

Question

I have an RMarkdown document outputting to HTML of the same form as the below example. What do I add where to apply unique CSS ids or classes to each plot output?

---
title: "RMarkdown"
author: "Me"
date: "Friday, March 27, 2015"
output:
  html_document:
    theme: null
    css: style.css
---

```{r plot1, echo=FALSE, warning=FALSE, message=FALSE}
library(ggplot2)
x <- ggplot(some_r_code)
print(x)
```

```{r plot2, echo=FALSE, warning=FALSE, message=FALSE}
y <- ggplot(some_more_r_code)
print(y)
```

I've read the info page at http://rmarkdown.rstudio.com/html_document_format.html that went a ways to answering this question but didn't get me there. I have a similar question referencing the material in that page in it's comment section, and would appreciate an answer on either.

Thanks!

Answer 1

You can tell knitr (which is used under the hood) with results="asis" to embed a chunk's output directly into the html. Within the chunk you can use cat to simply write a style tag including your css definitions:

```{r results="asis"}
cat("
<style>
h1 {
   color: red;
}
</style>
")
```

See http://yihui.name/knitr/options/#chunk_options for details.