diff --git a/README.Rmd b/README.Rmd
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--- a/README.Rmd
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@@ -311,6 +311,26 @@
 ---
 ```
 
+## Lightbox
+
+reveal.js 5.2 added a [lightbox](https://revealjs.com/lightbox/): any element on a slide can be turned into a clickable trigger that opens an image or video full-size in a dimmed overlay, without leaving the slide. This is useful for dense figures or tables that should stay readable in the slide layout but be inspectable at full size during a presentation.
+
+Add the corresponding data attribute to an image:
+
+```markdown
+![Figure](plot.png){data-preview-image}
+```
+
+or set it on knitr-generated figures via `out.extra`:
+
+````markdown
+```{r, out.extra='data-preview-image'}
+plot(cars)
+```
+````
+
+Clicking the image on the slide opens it full-size; `Esc` or another click closes the overlay.
+
 ## MathJax Equations
 
 By default [MathJax](http://www.mathjax.org/) scripts are included in reveal.js presentations for rendering LaTeX and MathML equations. You can use the `mathjax` option to control how MathJax is included: