diff --git a/packages/core/src/extensions/extension.ts b/packages/core/src/extensions/extension.ts
index 019e9002..72ee4e4f 100644
--- a/packages/core/src/extensions/extension.ts
+++ b/packages/core/src/extensions/extension.ts
@@ -39,6 +39,7 @@ import { defineMeowdownParagraph } from './paragraph.ts'
 import { definePendingReplacement } from './pending-replacement.ts'
 import { defineScrollToSelection } from './scroll-to-selection.ts'
 import { defineSelectDocBoundary } from './select-doc-boundary.ts'
+import { defineSystemSubstitutionGuard } from './system-substitution-guard.ts'
 import { defineTable } from './table.ts'
 import { defineViewAttributes } from './view-attributes.ts'
 import { defineWikilink } from './wikilink.ts'
@@ -77,6 +78,7 @@ function defineEditorExtensionImpl(options: EditorExtensionOptions) {
     defineClipboard(),
     defineScrollToSelection(),
     defineHiddenRunCaret(),
+    defineSystemSubstitutionGuard(),
     defineAtomMarkNavigation({
       marks: ATOM_SOURCE_MARK_NAMES.map((name) => ({ name, modes: ['hide', 'focus', 'show'] })),
     }),
diff --git a/packages/core/src/extensions/spell-check.ts b/packages/core/src/extensions/spell-check.ts
index a5bad38b..c5855378 100644
--- a/packages/core/src/extensions/spell-check.ts
+++ b/packages/core/src/extensions/spell-check.ts
@@ -117,6 +117,9 @@ function createSpellCheckPlugin(spellCheck: boolean) {
   })
 }
 
+/**
+ * @deprecated This will be removed in future versions.
+ */
 export function defineSpellCheckPlugin(spellCheck: boolean): PlainExtension {
   return definePlugin(createSpellCheckPlugin(spellCheck))
 }
diff --git a/packages/core/src/extensions/system-substitution-guard.ts b/packages/core/src/extensions/system-substitution-guard.ts
new file mode 100644
index 00000000..06801a0e
--- /dev/null
+++ b/packages/core/src/extensions/system-substitution-guard.ts
@@ -0,0 +1,41 @@
+import { definePlugin, Priority, withPriority, type PlainExtension } from '@prosekit/core'
+import { Plugin } from '@prosekit/pm/state'
+
+const EM_DASH = '\u{2014}'
+
+/**
+ * Block the macOS "smart dashes" rewrite. With the `spellcheck` attribute on,
+ * WebKit rewrites already-typed `--` near the caret into an em dash and
+ * delivers the rewrite as a cancelable `beforeinput` with
+ * `inputType: 'insertReplacementText'`. That corrupts Markdown such as the
+ * `-->` closing an image sizing comment, so cancel any replacement that
+ * inserts an em dash; word-level autocorrect passes through.
+ */
+export function defineSystemSubstitutionGuard(): PlainExtension {
+  const plugin = new Plugin({
+    props: {
+      handleDOMEvents: {
+        beforeinput: (view, event) => {
+          if (event.inputType !== 'insertReplacementText') return false
+
+          const replacement = event.dataTransfer?.getData('text/plain') || event.data || ''
+          if (!replacement.includes(EM_DASH)) return false
+
+          event.preventDefault()
+
+          // Restore the selection after the cancelation. See WebKit bug: https://bugs.webkit.org/show_bug.cgi?id=321420
+          const { doc, selection } = view.state
+          requestAnimationFrame(() => {
+            const { state } = view
+            if (view.isDestroyed || state.doc !== doc || state.selection.eq(selection)) return
+            view.dispatch(state.tr.setSelection(selection))
+          })
+
+          return true
+        },
+      },
+    },
+  })
+
+  return withPriority(definePlugin(plugin), Priority.highest)
+}
diff --git a/packages/react/src/components/editor-extensions.tsx b/packages/react/src/components/editor-extensions.tsx
index e2f3328d..e5dcdd92 100644
--- a/packages/react/src/components/editor-extensions.tsx
+++ b/packages/react/src/components/editor-extensions.tsx
@@ -13,7 +13,6 @@ import {
   definePlaceholder,
   defineReadonly,
   defineSearchStatusHandler,
-  defineSpellCheckPlugin,
   defineSubstitution,
   defineTagClickHandler,
   defineViewAttributes,
@@ -106,6 +105,13 @@ export function EditorExtensions({
     return editor.use(extension)
   }, [editor, editorClassName])
 
+  // Set spellCheck
+  useLayoutEffect(() => {
+    if (spellCheck == null) return
+    const extension = defineViewAttributes({ spellcheck: spellCheck ? 'true' : 'false' })
+    return editor.use(extension)
+  }, [editor, spellCheck])
+
   // Search has no latency requirement, so a slow device may skip the
   // intermediate values of a fast typist entirely. `literal` turns off
   // prosemirror-search's escape-sequence handling: a find bar takes text, so a
@@ -242,11 +248,5 @@ export function EditorExtensions({
     }, [wikilinkEnabled]),
   )
 
-  useExtension(
-    useMemo(() => {
-      return spellCheck == null ? null : defineSpellCheckPlugin(spellCheck)
-    }, [spellCheck]),
-  )
-
   return null
 }
diff --git a/website/src/app.tsx b/website/src/app.tsx
index ababcb5e..e2ab7935 100644
--- a/website/src/app.tsx
+++ b/website/src/app.tsx
@@ -9,12 +9,12 @@ import {
 import { getId } from '@ocavue/utils'
 import { clsx } from 'clsx/lite'
 import {
-  type CSSProperties,
   useCallback,
   useEffect,
   useLayoutEffect,
   useRef,
   useState,
+  type CSSProperties,
 } from 'react'
 
 import { DemoEditor } from './components/demo-editor.tsx'
@@ -160,7 +160,7 @@ Outline your thoughts with nested bullets. Hover a bullet that has children and
 
 Drop in an image and it renders right where you wrote it. Paste or drag one in to upload your own:
 
-Small images flow inline ![](https://static.photos/yellow/16x16/3) with the surrounding text.
+Small images flow inline ![](https://static.photos/yellow/16x16/3)<!--{"width":128,"height":128}--> with the surrounding text.
 
 Paste a YouTube or tweet link and it embeds itself. Undo once to get the plain link back: