Changes

• The math plugin now supports three typesetting libraries: KaTeX, MathJax 2 and MathJax 3. We continue to use MathJax 2 as our default so this is fully backwards compatible. Learn how to choose between typesetters and how to configure them in the docs at https://revealjs.com/math#typesetting-libraries (@burgerga in #2559).
• New event: beforeslidechange (#3003). This makes it possible to conditionally prevent navigations:

  // This prevents all slide changes
  Reveal.addEventListener( 'beforeslidechange', e => e.preventDefault() );
  

• New keyboard shortcut for skipping fragments while navigating: alt + ←/↑/→/↓.
• New API option for skipping fragments in directional navigation Reveal.right({ skipFragments: true }).
• Adds a beforeHighlight callback to the highlight plugin (@rajgoel in #3026).

  Reveal.initialize({ 
    highlight: {
      beforeHighlight: (hljs) => {
        // interact with highlight.js, for example to register a new language
      }
    } 
  })
  

• Code line numbers can now start from an offset (#3050). For example, this code block would begin its line numbering from 10: <code data-ln-start-from="10">.
• Better error messaging when the .reveal or .slides containers are missing #2217.

Fixes

• The last slide keyboard shortcut now works for looped presentations (#3007).
• Markdown code blocks can be turned into fragments (@nicojs in #2982).
• Unit tests can now run in Windows (@Vandivier in #3027).
• Restored support for base64 background images, broken since 4.1.1 (#2978).
• Fixes an issue that prevented presentations from looping when navigationMode was set to linear.
• Internal links leading to a slide with video/audio element will now correctly start media playback. This issue only affected mobile browsers.