Changes:

• Adds step-by-step code highlights! Step through multiple line highlights on the same code block.
• Adds postMessage callbacks. Makes it possible to use the postMessage API to invoke reveal.js methods with return values.
• The pacing timer functionality now accepts a total time for the whole presentation. Timing was previously worked out on a per-slide level. (#2400 by longtime reveal.js contributor @fghaas!)
• Background iframes no longer preload by default. They load when you arrive at the given slide. This unifies the behavior of in-slide and background iframes. Learn how to turn on preloading.
• The slide number format specified through slideNumber is now honored in PDF exports. (#2337 by @dougalsutherland) (4c557a5959b3ad909056df6cb46c5bf56a0da8ee)
• Adds data-fragment=<index> to any slide
  with fragments in it. This lets you target specific fragment states with CSS like section[data-fragment="2"] { ... }.
• Adds Reveal.getHorizontalSlides() and Reveal. getVerticalSlides() for getting all horizontal/vertical slides in a deck.
• Adds Reveal.hasHorizontalSlides() and Reveal. hasVerticalSlides() for checking whether or not a deck contains any horizontal or vertical slides.
• Adds mobileViewDistance configuration option. Mobile view distance was previously hardcoded at 2. (#2513 by @TuurDutoit)
• Adds allow="autoplay" to iframes to comply with Chrome's Autoplay Policy Changes (#2437 by @TehDmitry)
• Switches to CSS transforms to scale decks up on HDPI displays. Previous use of CSS zoom produced sharper results but led to side effects such as iframes not scaling with the deck content.
• Switches first/last slide keyboard shortcuts from ⌘←/⌘→ to Shift←/Shift→. The old shortcut conflicted with browser back/forward.
• Updates highlight.js from 9.11.0 to 9.18.0

Bug fixes:

• Fixes an issue where the navigation down-arrow was blocked by the progress bar (#2410 by @NoriSte).
• Fixes swipe navigation for decks with navigationMode set to linear (#2416 by @earboxer).
• Fixes vertical overflow in iPadOS Safari.
• Fixes inconsistent fragment slide animations by translating by a fixed unit.
• Fixes failing npm install because of outdated dependencies.
• Fixes exception when highlighting empty code blocks.

And more...