♿️(frontend) fix empty heading before section titles in HTML export

Avoid nested headings: full HTML already wraps content in h1–h6; unwrap

CHANGELOG.md

@@ -9,6 +9,8 @@ and this project adheres to
 ### Changed
 
 - 💄(frontend) improve comments highlights #1961
+- ♿️(frontend) fix empty heading before section titles in HTML export
+  #2125
 
 ## [v4.8.3] - 2026-03-23
 

src/frontend/apps/impress/src/features/docs/doc-export/utils_html.ts

@@ -145,8 +145,20 @@ export const improveHtmlAccessibility = (
   headingBlocks.forEach((block) => {
     const rawLevel = Number(block.getAttribute('data-level')) || 1;
     const level = Math.min(Math.max(rawLevel, 1), 6);
-    const heading = parsedDocument.createElement(`h${level}`);
-    moveChildNodes(block, heading);
+    const tag = `h${level}`;
+
+    const existingHeading = block.querySelector('h1, h2, h3, h4, h5, h6');
+    const heading = parsedDocument.createElement(tag);
+
+    if (existingHeading) {
+      if (existingHeading.className) {
+        heading.className = existingHeading.className;
+      }
+      moveChildNodes(existingHeading, heading);
+    } else {
+      moveChildNodes(block, heading);
+    }
+
     block.replaceWith(heading);
   });