For the record, this issue has been fixed upstream, thanks to Christophe 
and Hakim:
https://github.com/fab4100/reveal.js/commit/3e0b68ea80e3c123def3182ecb183febafae8961

On Tuesday, 28 September, 2021 at 10:36:04 pm UTC+5:30 John MacFarlane 
wrote:

>
> Looking at the HTML using the browser's Inspect feature, I
> see that reveal.js is adding a div devoted to backgrounds,
> which uses a background-image-hash attribute. This may be
> constructed improperly for data URIs; anyway, this is one
> thing to investigate here.
>
> John MacFarlane <j...-TVLZxgkOlNX2fBVCVOL8/A@public.gmane.org> writes:
>
> > Not sure what is happening here, but yes, an issue on the tracker
> > would be welcome.
> >
> > I note also that because the data-background-image attribute
> > is retained on the h1 element as well as being propagated to
> > the section element, we get two copies of the encoded image.
> > That is bad. See also https://github.com/jgm/pandoc/issues/5965
> >
> >
> > christophe dervieux <christoph...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> writes:
> >
> >> Hello, 
> >>
> >> Here is a reproducible example of the issue
> >>
> >> curl --output background.jpg -L '
> https://unsplash.com/photos/8OyKWQgBsKQ/download?force=true&w=1920'
> >>
> >> echo "# Slide {data-background-image='background.jpg'}" > slide.md
> >> echo "" >> slide.md
> >> echo "Content" >> slide.md
> >>
> >> # Not self contained 
> >> pandoc -t revealjs -s -o slide.html --metadata title="test" slide.md
> >>
> >> # Self contained
> >> pandoc -t revealjs --self-contained -o slide-self-contained.html 
> --metadata title="test" slide.md
> >>
> >> When using self-contained, the background image is no more working. 
> Pandoc 
> >> encodes to data: but it seems reveal.js does not know how to handle it. 
> It 
> >> split the encoded data: in two url component.
> >>
> >> It seems like a change in reveal 4 as using the same slide.md file with 
> >> different version seems to break. 
> >>
> >> What I tried: 
> >>
> >> - Pandoc 2.9.2.1 : last one compatible with reveal.js 3 I believe
> >> 
> >> pandoc.exe -t revealjs --self-contained -o slide-self-contained.html 
> --metadata title="test" -V revealjs-url=https://unpkg.com/reveal.js@3/ 
> slide.md
> >> 
> >> Works as expected 
> >> - Pandoc 2.10: First to use reveal.js 4
> >> 
> >> pandoc.exe -t revealjs --self-contained -o slide-self-contained.html 
> --metadata title="test" -V revealjs-url=https://unpkg.com/reveal.js@4/ 
> slide.md
> >> 
> >> 
> >> So it seems that something broke when upgrading. 
> >>
> >> Is this a known issue ?
> >> Should I open an issue in Pandoc to track ? I believe this could be 
> >> something in reveal.js not handling correctly encoded data: field.
> >>
> >> MANUAL still says (
> >> https://pandoc.org/MANUAL.html#background-in-reveal.js-and-beamer)
> >>
> >> Background images can be added to self-contained reveal.js slideshows 
> and 
> >> to beamer slideshows.
> >>
> >> But maybe self-contained and revealjs does not work well. 
> >>
> >> Thank you
> >> ​
> >>
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>

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