Native support for R `cli` package ANSI-styled output in knitr engine

Summary

R code using the cli package produces unstyled, plain text output in Quarto HTML documents. The ANSI escape sequences are stripped, losing all formatting (colors, bold, emphasis). While Quarto 1.7+ added ANSI support for Jupyter/Julia, R/knitr documents still lack native handling.

Background

The cli package is widely used in the R ecosystem (tidyverse, devtools, usethis, etc.). Many custom print methods leverage cli for informative output. Without native support, Quarto users see degraded output quality compared to RStudio console or terminal rendering.

Minimal Reproducible Example

Save as cli-demo.qmd and render with quarto render cli-demo.qmd:

---
title: "CLI Output Demo"
format: html
---

```{r}
#| echo: false
library(cli)

demo_behavior <- function() {
  cli_h1("Status Report")
  cli_alert_success("All tests passed")
  cli_alert_warning("Memory usage high")
  cli_alert_danger("Connection failed")
}

demo_behavior()
```

Output Comparison

Console Output (Expected)	Quarto Rendered Output (Actual)

Desired Output

Mirror RStudio with a light grey background for CLI output:

Or, just the regular CLI output:

Expected vs Actual Behavior

Aspect	Expected	Actual
Colors	Styled with appropriate colors (green ✔, yellow ⚠, red ✖)	Plain monochrome text
Formatting	Bold headers, styled alerts	No text styling
Icons	Unicode symbols rendered with color context	Symbols appear but lack styling
Background	Terminal-style grey background	White/default background

Current Workaround

Users must manually configure custom knitr hooks using fansi in every document:

```{r}
#| include: false
options(crayon.enabled = TRUE, cli.num_colors = 256)

ansi_aware_handler <- function(x, options) {
  paste0(
    '<pre class="r-output"><code>',
    fansi::sgr_to_html(x = htmltools::htmlEscape(x), warn = FALSE, term.cap = "256"),
    '</code></pre>'
  )
}

knitr::knit_hooks$set(
  output = ansi_aware_handler,
  message = ansi_aware_handler,
  warning = ansi_aware_handler,
  error = ansi_aware_handler
)
```

This is non-obvious, requires additional dependencies (fansi, htmltools), and must be repeated in every document.

[!NOTE]
The workaround produces separate <pre> blocks for each cli statement, which may result in extra vertical spacing. This is normal knitr behavior (one output block per cat()/message() call). If desired, CSS can be used to collapse consecutive output blocks:

```{=html}
<style>
pre.r-output {
  margin: 0;
  padding: 0;
}
pre.r-output + pre.r-output {
  border-top: none;
}
</style>
```

Proposed Solution

Add native ANSI processing for R/knitr output, similar to the fix in PR #7813. Also consider adding a feature flag or three for the new behavior:

1. Add a chunk option like ansi: true to enable ANSI-to-HTML conversion
2. Document-level YAML option such as execute: ansi-colors: true
3. Terminal-style background for ANSI output blocks (grey background to visually distinguish from regular output)

Related Issues

Issue	Title	Status
#5746	knitr: process ANSI color codes	Open
#3730	Support ANSI colour output with downlit	Open
#4204	ASCII escape codes should be stripped or processed to HTML	Closed (Jupyter only)
Discussion #3957	{cli} output not captured by quarto slides	Active

Environment

• Quarto version: 1.8.26
• R version: 4.5.2
• cli package version: 3.6.5
• OS: macOS 26.2