---
title: "Markdown collapsible section: details tags in pandoc"
slug: markdown-collapsible-section
excerpt: "See Test file and reader commands for the input and the pandoc lines behind these markdown collapsible section runs."
quickAnswer: "Pandoc's three commands sit next to the input in Test file and reader commands. Output for the first block is quoted under Bold text with no blank lines around it. Under Open attribute on the second block, the opening tag from the three runs appears. Line breaks near summary text are compared in Where the summary text landed. For the list and code output, see List and fence set off by blank lines. A rerun with added blank lines sits in Bold line padded with blank lines. Runs on a nested input are in Details nested inside details. For backticks in a summary, see Code span in the summary. A bullet holding a block is covered by Details block in a list item. One run with an extension removed is quoted in Switching markdown_in_html_blocks off. Grep steps are numbered in Counting tags with grep."
keyTakeaways:
  - "Under Bold text with no blank lines around it, the first block's output is quoted reader by reader."
  - "Check Where the summary text landed for the summary lines from the three runs."
  - "In Bold line padded with blank lines, a rerun of that block with blank lines added is quoted."
  - "Read Details block in a list item for one block placed under a bullet."
  - "Steps and counts for saved output files are numbered in Counting tags with grep."
publishedDate: 2026-10-21
dateModified: 2026-10-21
author: Bobby Huang
cluster: B
role: post
status: published
faq:
  - q: Why did my bold text stay as asterisks inside a details block?
    a: "Output for that block under three readers is quoted in Bold text with no blank lines around it."
  - q: Can one details block sit inside another?
    a: "The section Details nested inside details quotes the runs on a nested input."
  - q: Do backticks work in the summary line?
    a: "See Code span in the summary for the three outputs."
  - q: Can markdown_in_html_blocks be switched off?
    a: "That single run is quoted in Switching markdown_in_html_blocks off."
related:
  - raw-html-in-markdown
  - markdown-code-blocks
  - markdown-comments
---

In these tests, a markdown collapsible section means an HTML `<details>` element typed into Markdown source, with a `<summary>` line as its first child. In these pandoc 3.8.3 runs, `markdown` treated text inside those tags one way, and `commonmark` and `commonmark_x` treated it another.

The commands below were run once apiece on a macOS computer, using the readers `commonmark_x`, `commonmark` and `markdown`. What follows is pandoc's HTML, copied from the logs. How any program might display that HTML to a person was never tested here.

## Test file and reader commands

The input is `details.md`. Running `awk 'END{print NR}' details.md` printed `20`, so the file has 20 lines. Here it is as `cat details.md` printed it:

````text
<details>
<summary>Test log</summary>
**3 tests failed**
</details>

<details open>
<summary>Open by default</summary>

- item one

</details>

<details>
<summary>Fence inside</summary>

```bash
npm test
```

</details>
````

The file holds three `<details>` blocks. Block one has no blank line anywhere inside it. Block two carries an `open` attribute plus a list holding one item, and blank lines sit on both sides of that list. Block three holds a fenced `bash` block, also set off by blank lines.

One command per reader converted it:

```sh
pandoc -f markdown -t html details.md
pandoc -f commonmark -t html details.md
pandoc -f commonmark_x -t html details.md
```

Each of the three runs returned rc 0 and printed an HTML fragment.

## Bold text with no blank lines around it

Block one came out of `markdown` like this:

```html
<details>
<summary>
Test log
</summary>
<strong>3 tests failed</strong>
</details>
```

With `commonmark`, the block read:

```html
<details>
<summary>Test log</summary>
**3 tests failed**
</details>
```

Those four lines also came from `commonmark_x`, byte for byte. So the `markdown` reader turned the double asterisks into a `<strong>` element, while `commonmark` and `commonmark_x` left the asterisks in place as plain characters.

The extension lists match this divide. In the list `pandoc --list-extensions` printed for `markdown`, line 42 reads `+markdown_in_html_blocks`. A `grep -c` for that name counted 1 line in the `markdown` list, 0 in the `commonmark` list and 0 in the `commonmark_x` list. It is a pandoc extension, and it does not appear in either CommonMark reader's list at all.

Why blank lines matter around HTML blocks is explained in [raw HTML in Markdown](raw-html-in-markdown.md); this page sticks with output.

## Open attribute on the second block

Block two opens with `<details open>`. In the three runs, the output kept that tag as `<details open>`, attribute and all.

Pandoc's `raw_html` extension is switched on in the three lists: `+raw_html` sits at line 53 of the `markdown` list and at line 18 of the `commonmark` and `commonmark_x` lists. That fits the tag passing through. The test stopped at the HTML pandoc produced.

## Where the summary text landed

Line breaks inside `<summary>` came out differently under `markdown` than under `commonmark` and `commonmark_x`. The `markdown` run moved the summary text to a separate line, sandwiched by `<summary>` and `</summary>`:

```html
<summary>
Open by default
</summary>
```

Under `commonmark` and `commonmark_x`, it stayed on one line as `<summary>Open by default</summary>`, matching the input.

The `markdown` output for `details.md` has no empty lines, while the `commonmark` output and the `commonmark_x` output have four empty lines apiece.

## List and fence set off by blank lines

Blocks two and three gave matching bodies under the three readers. The list came out as:

```html
<ul>
<li>item one</li>
</ul>
```

For the fenced `bash` block, pandoc wrote:

```html
<div class="sourceCode" id="cb1"><pre
class="sourceCode bash"><code class="sourceCode bash"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="ex">npm</span> test</span></code></pre></div>
```

The three readers wrote these lines identically. For these two blocks, the summary lines and the empty lines were where `markdown` parted ways with `commonmark` and `commonmark_x`. Fence syntax is the subject of [Markdown code blocks](markdown-code-blocks.md).

## Bold line padded with blank lines

An extra script retried block one after padding the bold line with one empty line above and one below. The file `details-blank.md` is 6 lines long by the same `awk` count:

```text
<details>
<summary>Test log</summary>

**3 tests failed**

</details>
```

The `markdown` reader returned:

```html
<details>
<summary>
Test log
</summary>
<p><strong>3 tests failed</strong></p>
</details>
```

From `commonmark` came the lines below, and `commonmark_x` printed them identically:

```html
<details>
<summary>Test log</summary>

<p><strong>3 tests failed</strong></p>
</details>
```

Once those two blank lines were in, the three readers each wrapped the bold text in `<p><strong>`. The summary line still differed: split over three lines under `markdown`, kept whole under `commonmark` and `commonmark_x`.

## Details nested inside details

The file `details-nested.md` has 11 lines and puts one block inside another, with blank lines between the parts:

```text
<details>
<summary>Outer</summary>

<details>
<summary>Inner</summary>

Inner **text**

</details>

</details>
```

Output from `markdown`:

```html
<details>
<summary>
Outer
</summary>
<details>
<summary>
Inner
</summary>
<p>Inner <strong>text</strong></p>
</details>
</details>
```

Pandoc wrote the following for `commonmark`, and the same for `commonmark_x`:

```html
<details>
<summary>Outer</summary>

<details>
<summary>Inner</summary>

<p>Inner <strong>text</strong></p>
</details>

</details>
```

Both nesting levels survived in the three runs, and each reader turned the inner bold text into `<strong>`.

## Code span in the summary

The 6-line file `details-codespan.md` puts a code span on the summary line:

```text
<details>
<summary>Run `npm test`</summary>

Done.

</details>
```

Under `markdown`, the summary came out as `Run <code>npm test</code>` on its own line:

```html
<details>
<summary>
Run <code>npm test</code>
</summary>
<p>Done.</p>
</details>
```

Under `commonmark` and `commonmark_x`, the backticks stayed as typed:

```html
<details>
<summary>Run `npm test`</summary>

<p>Done.</p>
</details>
```

Put plainly, backticks on the summary line produced `<code>` under `markdown`, while `commonmark` and `commonmark_x` kept them as backtick text.

## Details block in a list item

The file `details-list.md` is 8 lines and indents a block under a bullet:

```text
- Step one

  <details>
  <summary>More</summary>

  Step **detail**

  </details>
```

The `markdown` reader printed:

```html
<ul>
<li><p>Step one</p>
<details>
<summary>
<p>More</p>
</summary>
<p>Step <strong>detail</strong></p>
</details></li>
</ul>
```

From `commonmark`, with `commonmark_x` matching it line for line:

```html
<ul>
<li><p>Step one</p>
<details>
<summary>More</summary>

<p>Step <strong>detail</strong></p>
</details>
</li>
</ul>
```

Under `markdown`, the summary text gained a `<p>` wrapper inside the list item. Under `commonmark` and `commonmark_x`, it did not. The bold body text became `<strong>` in the three runs.

## Switching markdown_in_html_blocks off

A reader name can carry a minus sign followed by an extension name. The extra script applied that form to `details.md`:

```sh
pandoc -f markdown-markdown_in_html_blocks -t html details.md
```

It returned rc 0 and printed:

````html
<details>
<summary>Test log</summary>
**3 tests failed**
</details>
<details open>
<summary>Open by default</summary>

- item one

</details>
<details>
<summary>Fence inside</summary>

```bash
npm test
```

</details>
````

With that extension removed, `markdown` kept the asterisks, the list dash and the backtick fence as plain text inside the three blocks. The summary lines stayed whole too. The empty lines between the blocks were gone, as in the default `markdown` run.

## Counting tags with grep

The extra script issued these commands after a `cd` into the test folder. Replace the file name with yours to repeat them.

```sh
pandoc -f markdown -t html details.md -o details-out-markdown.html
pandoc -f commonmark -t html details.md -o details-out-commonmark.html
pandoc -f commonmark_x -t html details.md -o details-out-commonmark_x.html
grep -c '<strong>' details-out-markdown.html
grep -c '<strong>' details-out-commonmark.html
grep -c '<strong>' details-out-commonmark_x.html
grep -c '<details open>' details-out-markdown.html
grep -c '<details open>' details-out-commonmark.html
grep -c '<details open>' details-out-commonmark_x.html
```

1. Run the `pandoc` lines, which write one output file per reader. The log records rc 0 for those three runs.
2. Count `<strong>` lines. The counts were `1` for `markdown`, `0` for `commonmark` and `0` for `commonmark_x`.
3. Count `<details open>` lines. Every file gave `1` here.

A count of `0` in step 2 lines up with the asterisks that `commonmark` and `commonmark_x` left in block one. The sibling post on [diff fences in Markdown](markdown-diff-code-block.md) puts the same three readers to work on a `diff` fence.

## Where Sashmark fits

If Sashmark turns out as planned, you could fix a typo by clicking into the rendered page and typing. On save, the Round-Trip step is designed to write that single fix back and leave each neighboring byte alone. Changes View, a second planned feature, would flag sections an agent revised behind your back, whether or not the folder sits under git.

It is in development and has not been released. The app itself has no AI features. See the [planned features](/features), the [help pages](/help) and the [roadmap](/roadmap) for where each part stands.

More at [sashmark.com](https://www.sashmark.com/).