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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