diff --git a/services/web/frontend/js/features/source-editor/languages/markdown/index.ts b/services/web/frontend/js/features/source-editor/languages/markdown/index.ts
index c4d987aea1..d80c7e3bfd 100644
--- a/services/web/frontend/js/features/source-editor/languages/markdown/index.ts
+++ b/services/web/frontend/js/features/source-editor/languages/markdown/index.ts
@@ -1,7 +1,8 @@
 import { markdown as markdownLanguage } from '@codemirror/lang-markdown'
+import { yamlFrontmatter } from '@codemirror/lang-yaml'
 import { shortcuts } from './shortcuts'
 import { languages } from '../index'
-import { Strikethrough, Frontmatter } from '@lezer/markdown'
+import { Strikethrough } from '@lezer/markdown'
 import {
   HighlightStyle,
   LanguageSupport,
@@ -10,23 +11,25 @@ import {
 import { tags } from '@lezer/highlight'
 import { markdownDocumentOutline } from './document-outline'
 import { quartoCompletions } from './complete'
-import { yamlFrontmatterHighlighting } from './yaml-frontmatter'
 
 export const markdown = () => {
-  const { language, support } = markdownLanguage({
+  const mdLS = markdownLanguage({
     codeLanguages: languages,
-    // Frontmatter teaches the Markdown grammar to recognise the leading ---/---
-    // block as a YAML header rather than mis-parsing it as Setext headings.
-    extensions: [Strikethrough, Frontmatter],
+    extensions: [Strikethrough],
   })
 
-  return new LanguageSupport(language, [
-    support,
+  // yamlFrontmatter wraps the Markdown language so that the leading ---/---
+  // block is parsed as real YAML (with full Lezer YAML highlighting) and the
+  // rest of the document is still parsed as Markdown.  It replaces our
+  // previous regex-based ViewPlugin and the manual Frontmatter extension.
+  const quartoLS = yamlFrontmatter({ content: mdLS })
+
+  return new LanguageSupport(quartoLS.language, [
+    quartoLS.support,
     shortcuts(),
     syntaxHighlighting(markdownHighlightStyle),
     markdownDocumentOutline,
-    language.data.of({ autocomplete: quartoCompletions }),
-    yamlFrontmatterHighlighting,
+    mdLS.language.data.of({ autocomplete: quartoCompletions }),
   ])
 }
 
diff --git a/services/web/frontend/js/features/source-editor/languages/markdown/yaml-frontmatter.ts b/services/web/frontend/js/features/source-editor/languages/markdown/yaml-frontmatter.ts
deleted file mode 100644
index ea7f11f521..0000000000
--- a/services/web/frontend/js/features/source-editor/languages/markdown/yaml-frontmatter.ts
+++ /dev/null
@@ -1,120 +0,0 @@
-import { RangeSetBuilder } from '@codemirror/state'
-import {
-  Decoration,
-  DecorationSet,
-  EditorView,
-  ViewPlugin,
-  ViewUpdate,
-} from '@codemirror/view'
-
-// Matches YAML boolean/null literals
-const YAML_BOOL_RE = /^(true|false|yes|no|on|off|null|~)$/i
-// Matches plain integers and floats
-const YAML_NUMBER_RE = /^-?(?:0x[\da-fA-F]+|0o[0-7]+|\d+(?:\.\d*)?|\.\d+)(?:[eE][+-]?\d+)?$/
-
-function valueClass(value: string): string {
-  if (value.startsWith('"') || value.startsWith("'")) return 'tok-string'
-  if (YAML_BOOL_RE.test(value)) return 'tok-keyword'
-  if (YAML_NUMBER_RE.test(value)) return 'tok-number'
-  return 'tok-attributeValue'
-}
-
-// Find the first ':' that acts as a YAML key-value separator — it must be
-// followed by a space, tab, or end-of-string (not '/' as in URLs).
-function findKeyColon(text: string): number {
-  let inSingle = false
-  let inDouble = false
-  for (let i = 0; i < text.length; i++) {
-    const c = text[i]
-    if (c === "'" && !inDouble) { inSingle = !inSingle; continue }
-    if (c === '"' && !inSingle) { inDouble = !inDouble; continue }
-    if (inSingle || inDouble) continue
-    if (c === ':') {
-      const next = text[i + 1]
-      if (next === undefined || next === ' ' || next === '\t') return i
-    }
-  }
-  return -1
-}
-
-function buildDecorations(view: EditorView): DecorationSet {
-  const doc = view.state.doc
-  const builder = new RangeSetBuilder<Decoration>()
-
-  if (doc.lines < 2) return builder.finish()
-
-  // Only activate when the document starts with a YAML frontmatter block
-  if (doc.line(1).text.trim() !== '---') return builder.finish()
-
-  let fmEndLine = -1
-  for (let n = 2; n <= doc.lines; n++) {
-    const lineText = doc.line(n).text.trim()
-    if (lineText === '---' || lineText === '...') {
-      fmEndLine = n
-      break
-    }
-  }
-  if (fmEndLine < 0) return builder.finish()
-
-  // Color the opening '---' delimiter
-  const openLine = doc.line(1)
-  builder.add(openLine.from, openLine.to, Decoration.mark({ class: 'tok-comment' }))
-
-  // Decorate content lines 2..(fmEndLine - 1)
-  for (let n = 2; n < fmEndLine; n++) {
-    const line = doc.line(n)
-    const text = line.text
-    const trimmed = text.trimStart()
-    if (!trimmed) continue
-
-    const indent = text.length - trimmed.length
-    const base = line.from + indent
-
-    // YAML comment
-    if (trimmed[0] === '#') {
-      builder.add(base, line.to, Decoration.mark({ class: 'tok-comment' }))
-      continue
-    }
-
-    // Key: value — use tok-typeName for keys (35/41 theme coverage, distinct
-    // colour in most themes) and type-appropriate classes for values.
-    const colonPos = findKeyColon(trimmed)
-    if (colonPos <= 0) continue
-
-    const key = trimmed.slice(0, colonPos).trimEnd()
-    if (key) {
-      builder.add(base, base + key.length, Decoration.mark({ class: 'tok-typeName' }))
-    }
-
-    const afterColon = trimmed.slice(colonPos + 1)
-    const leadingSpace = afterColon.length - afterColon.trimStart().length
-    const value = afterColon.trimStart().trimEnd()
-    if (value) {
-      const valFrom = base + colonPos + 1 + leadingSpace
-      builder.add(valFrom, valFrom + value.length, Decoration.mark({ class: valueClass(value) }))
-    }
-  }
-
-  // Color the closing '---' / '...' delimiter
-  const closeLine = doc.line(fmEndLine)
-  builder.add(closeLine.from, closeLine.to, Decoration.mark({ class: 'tok-comment' }))
-
-  return builder.finish()
-}
-
-// Highlights YAML key/value/comment lines inside a Quarto/Pandoc frontmatter
-// block (the region delimited by leading and trailing '---' lines).
-export const yamlFrontmatterHighlighting = ViewPlugin.fromClass(
-  class {
-    decorations: DecorationSet
-    constructor(view: EditorView) {
-      this.decorations = buildDecorations(view)
-    }
-    update(update: ViewUpdate) {
-      if (update.docChanged || update.viewportChanged) {
-        this.decorations = buildDecorations(update.view)
-      }
-    }
-  },
-  { decorations: v => v.decorations }
-)
diff --git a/services/web/package.json b/services/web/package.json
index 7ae7a2ba51..a86bf268e7 100644
--- a/services/web/package.json
+++ b/services/web/package.json
@@ -206,6 +206,7 @@
     "@codemirror/commands": "6.10.1",
     "@codemirror/lang-markdown": "6.5.0",
     "@codemirror/lang-python": "6.2.1",
+    "@codemirror/lang-yaml": "6.1.3",
     "@codemirror/language": "6.12.1",
     "@codemirror/lint": "6.9.2",
     "@codemirror/search": "6.5.8",