Create epub version 3.0 in bookdown?

Hi everyone! I'm in bookdown, trying to output an epub that is compatible with epub 3.0, and I'm totally stuck.

In my _output.yaml I have:

epub_version: epub3

and it compiles and exports fine but the online publisher is rejecting the file, with the following errors:

ERROR(RSC-005): .tmp.tmpzwj5eul4.epub/EPUB.nav.xhtml(19,9): Error while parsing file: element "ol" incomplete; missing required element "li"

ERROR(RSC-005): .tmp.tmpzwj5eul4.epub/EPUB.text.ch003.xhtml(20,9): Error while parsing file: element "center" not allowed here; expected the element end-tag, text, element "a", "abbr", "address", "area", "article", "aside", "audio", "b", "bdi", "bdo", "blockquote", "br", "button", "canvas", "cite", "code", "data", "datalist", "del", "details", "dfn", "dialog", "div", "dl", "em", "embed", "epub:switch", "epub:trigger", "fieldset", "figure", "footer", "form", "h1", "h2", "h3", "h4", "h5", "h6", "header", "hgroup", "hr", "i", "iframe", "img", "input", "ins", "kbd", "label", "link", "main", "map", "mark", "menu", "meta", "meter", "nav", "ns1:math", "ns2:svg", "object", "ol", "output", "p", "picture", "pre", "progress", "q", "ruby", "s", "samp", "script", "section", "select", "small", "span", "strong", "sub", "sup", "table", "template", "textarea", "time", "u", "ul", "var", "video" or "wbr" (with xmlns:ns1="" xmlns:ns2="") or an element from another namespace

I have also tried adding:

epub_book(fig_width = 5, fig_height = 4, dev = "png", 
  fig_caption = TRUE, number_sections = TRUE, toc = FALSE, 
  toc_depth = 3, stylesheet = NULL, cover_image = NULL, 
  metadata = NULL, chapter_level = 1, epub_version = c("epub3", 
    "epub"), md_extensions = NULL, pandoc_args = NULL, 
  template = "default")

in the .yaml and the index.rmd but it just creates errors, I assume because I am putting it in the wrong place? Thank you, community.

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Update: it looks like this is actually a problem with online epub validators becoming more strict about their processes. I found a blog post about it here:

To figure this out, I created a new bookdown project, rendered the book using the default values, and uploaded the epub to two different epub validator websites (like, which both gave the same error shown in the blog post linked above: "Line 19, position 9: Error while parsing file: element "ol" incomplete; missing required element "li""

I also confirmed that the default version for the epub function is epub3, so that wasn't the problem at all.

Thanks for going with me on this journey.

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