works with video, defaults to (same as for images)

README.md

@@ -578,6 +578,7 @@ Automatically plays a full size video behind the slide.
 | data-background-video        |         | A single video source, or a comma separated list of video sources. |
 | data-background-video-loop   | false   | Flags if the video should play repeatedly. |
 | data-background-video-muted  | false   | Flags if the audio should be muted. |
+| data-background-size         | cover   | Use `cover` for full screen and some cropping or `contain` for letterboxing. |
 
 ```html
 <section data-background-video="https://s3.amazonaws.com/static.slid.es/site/homepage/v1/homepage-video-editor.mp4,https://s3.amazonaws.com/static.slid.es/site/homepage/v1/homepage-video-editor.webm" data-background-video-loop data-background-video-muted>

css/reveal.css

@@ -865,7 +865,13 @@ body {
   max-width: none;
   max-height: none;
   top: 0;
-  left: 0; }
+  left: 0;
+  -o-object-fit: cover;
+     object-fit: cover; }
+
+.reveal .slide-background[data-background-size="contain"] video {
+  -o-object-fit: contain;
+     object-fit: contain; }
 
 /* Immediate transition style */
 .reveal[data-background-transition=none] > .backgrounds .slide-background,

css/reveal.scss

@@ -898,6 +898,10 @@ body {
 	max-height: none;
 	top: 0;
 	left: 0;
+	object-fit: cover;
+}
+	.reveal .slide-background[data-background-size="contain"] video {
+		object-fit: contain;
 	}
 
 /* Immediate transition style */

js/reveal.js

@@ -884,6 +884,7 @@
 
 		// Additional and optional background properties
 		if( data.backgroundSize ) element.style.backgroundSize = data.backgroundSize;
+		if( data.backgroundSize ) element.setAttribute( 'data-background-size', data.backgroundSize );
 		if( data.backgroundColor ) element.style.backgroundColor = data.backgroundColor;
 		if( data.backgroundRepeat ) element.style.backgroundRepeat = data.backgroundRepeat;
 		if( data.backgroundPosition ) element.style.backgroundPosition = data.backgroundPosition;