Fix multi-line subtitle escaping its CSS class in HTML output

src/resources/filters/normalize/normalize.lua

@@ -20,7 +20,16 @@ local authors = require 'modules/authors'
 local license = require 'modules/license'
 local shortcode_ast = require 'modules/astshortcode'
 
-function normalize_filter() 
+-- Convert block-level metadata to inline for fields rendered inside <p> tags.
+-- Multi-line YAML values produce MetaBlocks (Para), which nest <p> in <p> — invalid HTML5.
+local function ensureMetaInlines(meta, field)
+  local val = meta[field]
+  if val ~= nil and quarto.utils.type(val) == "Blocks" then
+    meta[field] = quarto.utils.as_inlines(val)
+  end
+end
+
+function normalize_filter()
   return {
     Meta = function(meta)
       -- normalizes the author/affiliation metadata
@@ -32,6 +41,9 @@ function normalize_filter()
       -- normalizes the license metadata
       normalized = license.processLicenseMeta(normalized)
 
+      -- Convert block-level metadata to inline for fields rendered in <p> tags
+      ensureMetaInlines(normalized, "subtitle")
+
       -- for JATs, forward keywords or categories to tags
       if _quarto.format.isJatsOutput() then
         if normalized.tags == nil then

tests/docs/smoke-all/2026/02/16/issue-13827-multiline.qmd

@@ -0,0 +1,24 @@
+---
+title: Test Title
+subtitle: |
+  First Part
+
+  Second Part
+format:
+  html: default
+  revealjs: default
+_quarto:
+  tests:
+    html:
+      ensureHtmlElementContents:
+        selectors: ['p.subtitle']
+        matches: ['First Part']
+    revealjs:
+      ensureHtmlElementContents:
+        selectors: ['p.subtitle']
+        matches: ['First Part']
+---
+
+## Test Content
+
+This tests that a multi-paragraph | scalar subtitle stays inside the subtitle class.

tests/docs/smoke-all/2026/02/16/issue-13827.qmd

@@ -0,0 +1,22 @@
+---
+title: Test Title
+subtitle: |
+  A Single Line Subtitle
+format:
+  html: default
+  revealjs: default
+_quarto:
+  tests:
+    html:
+      ensureHtmlElementContents:
+        selectors: ['p.subtitle']
+        matches: ['A Single Line Subtitle']
+    revealjs:
+      ensureHtmlElementContents:
+        selectors: ['p.subtitle']
+        matches: ['A Single Line Subtitle']
+---
+
+## Test Content
+
+This tests that a single-line | scalar subtitle stays inside the subtitle class.