multi-line subtitle is rendered outside the `subtitle` class

[herosi]

I have:

• searched the issue tracker for similar issues
• installed the latest version of Quarto CLI
• formatted my issue following the Bug Reports guide

Bug description

When multi-line subtitle is used like the following code, the content is rendered outside the subtitle class like the following figure.

---
format:
  revealjs
title: |
  aaa\
  bbb
subtitle: |
  xxx\
  yyy
---

Steps to reproduce

No response

Actual behavior

When multi-line subtitle is used like the code above, the content is rendered outside the subtitle class like the figure above.

Expected behavior

The content should be inside the subtitle class.

Your environment

OS: Windows 11
IDE: VS Code

Quarto check output

quarto check
Quarto 1.8.26
[>] Checking environment information...
      Quarto cache location: C:\Users\xxxx\AppData\Local\quarto
[>] Checking versions of quarto binary dependencies...
      Pandoc version 3.6.3: OK
      Dart Sass version 1.87.0: OK
      Deno version 2.3.1: OK
      Typst version 0.13.0: OK
[>] Checking versions of quarto dependencies......OK
[>] Checking Quarto installation......OK
      Version: 1.8.26     
      Path: C:\Users\xxxx\AppData\Local\Programs\Quarto\bin
      CodePage: 932

[>] Checking tools....................OK
      TinyTeX: (not installed)
      Chromium: (not installed)

(|) Checking LaTeX........(/) Checking LaTeX........(-) Checking LaTeX........(\) Checking LaTeX........
[>] Checking LaTeX....................OK
      Tex:  (not detected)

(|) Checking Chrome Headle(/) Checking Chrome Headle[>] Checking Chrome Headless....................OK  
      Using: Chrome found on system
      Path: C:\Program Files (x86)\Microsoft\Edge\Application\msedge.exe      
      Source: Windows Registry

(|) Checking basic markdow(/) Checking basic markdow(-) Checking basic markdow(\) Checking basic markdow(|) Checking basic markdow(/) Checking basic markdow(-) Checking basic markdow(\) Checking basic markdow(|) Checking basic markdow(/) Checking basic markdow(-) Checking basic markdow(\) Checking basic markdow(|) Checking basic markdow(/) Checking basic markdow(-) Checking basic markdow(\) Checking basic markdow(|) Checking basic markdow[>] Checking basic markdown render....OK

(|) Checking Python 3 inst(/) Checking Python 3 inst(-) Checking Python 3 inst(\) Checking Python 3 inst(|) Checking Python 3 inst(/) Checking Python 3 inst(-) Checking Python 3 in[>] Checking Python 3 installation....OK
      Version: 3.14.2
      Path: C:/Users/xxxx/AppData/Local/Python/pythoncore-3.14-64/python.exe
      Jupyter: 5.9.1
      Kernels: python3

(\) Checking Jupyter engine render....C:\Users\xxxx\AppData\Local\Programs\Quarto\share\jupyter\jupyter.py:28: DeprecationWarning: 'asyncio.WindowsSelectorEventLoopPolicy' is deprecated and slated for removal in Python 3.16      
  asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())    
C:\Users\xxxx\AppData\Local\Programs\Quarto\share\jupyter\jupyter.py:28: DeprecationWarning: 'asyncio.set_event_loop_policy' is deprecated and slated for removal in Python 3.16
  asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())    
[>] Checking Jupyter engine render....OK

[>] Checking R installation...........(None)

      Unable to locate an installed version of R.
      Install R from https://cloud.r-project.org/

[mcanouil]

Thanks, the issue seems to be for all HTML formats.
It seems it's something related to having <p ...><p>...</p></p> nested, so I suspect a post-processing step doing this bad "cleaning".

---
format: html
title: "Issue 13827"
paragraph: |
  111\
  222
template-partials:
  - title-block.html
---

with title-block.html

<header id="title-block-header">
  <p class="paragraph">$paragraph$</p>
</header>

will lead to:

<header id="title-block-header">
    <h1 class="title display-7">Issue</h1>
    <p class="paragraph"></p><p>111<br>222</p><p></p>
</header>

If you change the partial to h5, then the value of paragraph is properly included.

<header id="title-block-header">
  <h5 class="paragraph">$paragraph$</h5>
</header>

[mcanouil]

Actually, you don't even need to use metadata to see this behaviour.

Using plain HTML (not recommended) in Markdown will result in the same behaviour:

---
format: html
---

<p class="paragraph"><p>111<br>222</p></p>

leads to:

...
<p class="paragraph">
</p><p>
111<br>222
</p>
<p></p>
...

Same as using raw block:

---
format: html
---

```{=html}
<p class="paragraph"><p>111<br>222</p></p>
```

Pandoc does not seem to be the issue:

echo '<p class="paragraph"><p>111<br>222</p></p>' | quarto pandoc --from markdown --to html

<p class="paragraph">
<p>
111<br>222
</p>
</p>

[herosi]

Great analysis! Thanks!

[cscheid]

(I suspect the HTML postprocessor here, but haven't verified.)

[cderv]

I've been looking into this to understand the root cause (with claude's help). Here are my findings.

Root cause chain:

1. The multi-line YAML value (using | literal block scalar) is parsed by Pandoc as MetaBlocks containing a Para element. You can see this with:

   echo -e "---\nsubtitle: |\n  xxx\\\n  yyy\n---\n" | quarto pandoc -f markdown -t json | jq '.meta.subtitle.t'
   # => "MetaBlocks"

   Single-line values produce MetaInlines instead — no wrapping <p>.

2. The title-block templates wrap $subtitle$ in <p class="subtitle">$subtitle$</p>. When $subtitle$ is MetaBlocks, Pandoc renders it as <p>xxx<br>yyy</p>, producing nested <p> tags — invalid HTML5.

3. Any HTML5-compliant parser — whether a browser or Quarto's deno-dom post-processing step — auto-closes the outer <p> when it encounters the inner <p>, producing: <p class="subtitle"></p><p>xxx<br>yyy</p><p></p>. The content escapes the .subtitle class.

The root issue is that Pandoc's template expansion produces invalid HTML: block-level metadata content (with <p> wrapping from MetaBlocks) gets placed inside a <p> element, creating nested <p> tags that the HTML5 spec forbids. This would break in any browser regardless of post-processing.

Possible fix direction: a Lua filter that converts MetaBlocks to MetaInlines using pandoc.utils.blocks_to_inlines() for specific fields that are placed inside <p> elements in templates (subtitle, author, date). Quick proof of concept:

-- fix-filter.lua
function Meta(meta)
  for key, val in pairs(meta) do
    if pandoc.utils.type(val) == "Blocks" then
      meta[key] = pandoc.Inlines(pandoc.utils.blocks_to_inlines(val))
    end
  end
  return meta
end

# Without filter:
<p class="subtitle"><p>xxx<br />yyy</p></p>

# With filter:
<p class="subtitle">xxx<br />yyy</p>

Note: a blanket conversion of all MetaBlocks would break fields like abstract that are intentionally block-level and wrapped in <div> containers (it would flatten multi-paragraph content). So the fix needs to be surgical — targeting only fields that are semantically inline and placed in <p> elements.

[cscheid]

Ok, so this goes away in q2 (or at least we can fix it in our own implementation).