diff --git a/content/posts/how-i-convert-pdfs-to-epub.adoc b/content/posts/how-i-convert-pdfs-to-epub.adoc
index b3231bc..44648a4 100644
--- a/content/posts/how-i-convert-pdfs-to-epub.adoc
+++ b/content/posts/how-i-convert-pdfs-to-epub.adoc
@@ -24,6 +24,7 @@ toc: true
 :url-helicon-type: https://www.heliconbooks.com/?id=blog&postid=EPUB3Footnotes
 :url-epub-spec: https://www.w3.org/publishing/epub/epub-contentdocs.html
 :url-ap-semantics: https://www.accessiblepublishing.ca/epub-semantic-aria-roles/
+:url-epub3-rendering: http://idpf.org/forum/topic-623
 :wp-pdf: https://en.wikipedia.org/wiki/PDF
 :wp-epub: https://en.wikipedia.org/wiki/EPUB
 :wp-xhtml: https://en.wikipedia.org/wiki/XHTML
@@ -60,11 +61,11 @@ Calibre creates EPUB 2 books by default. Convert the book to EPUB
 3footnote:[EPUB 3 introduces many accessibility features, see
 link:{url-ap-semantics}[AccessiblePublishing: EPUB Semantics, ARIA Roles, &
 Metadata] for details.] by clicking menu:Tools[Upgrade book internals]. This
-will, among other things, convert `toc.ncx` to `nav.xhtml`. To support e-book
-readers which can't handle EPUB 3 yet, re-create `toc.ncx` as an empty file. It
-is filled automatically when you create the table of contents. Open
-`metadata.opf` and replace `<spine>` with `<spine toc="id1">` (`id1` is the ID
-of `toc.ncx`, defined a few lines above).
+will, among other things, convert `toc.ncx` to `nav.xhtml`. To
+link:{url-epub3-rendering}[support e-book readers which can't handle EPUB 3
+yet], re-create `toc.ncx` as an empty file. It is filled automatically when you
+create the table of contents. Open `metadata.opf` and replace `<spine>` with
+`<spine toc="id1">` (`id1` is the ID of `toc.ncx`, defined a few lines above).
 
 You start with a single XHTML file, `start.xhtml`. I always use that for the
 title page, the copyright notice and so on. You can force a page break to