only offset slides vertically when center option is true

css/reveal.css

@@ -370,7 +370,7 @@ body {
 }
 
 .reveal table {
-  margin: auto;
+  margin: 0 auto;
   border-collapse: collapse;
   border-spacing: 0;
 }
@@ -594,7 +594,7 @@ body {
 	display: none;
 	position: absolute;
 	width: 100%;
-	padding: 20px 0px;
+	padding: 0px;
 
 	z-index: 10;
 	line-height: 1.2em;
@@ -619,6 +619,49 @@ body {
 						opacity 800ms cubic-bezier(0.260, 0.860, 0.440, 0.985);
 }
 
+.reveal .slides>section.visible,
+.reveal .slides>section>section.visible {
+	display: block;
+}
+
+.reveal.center .slides {
+	top: -40px;
+}
+
+.reveal.center .slides,
+.reveal.center .slides section {
+	height: 100%;
+}
+
+.reveal.center .slides section:not(.stack) {
+	display: -webkit-box;
+	display: -webkit-flex;
+	display: -moz-box;
+	display: -moz-flex;
+	display: -ms-flexbox;
+	display: flex;
+
+	-webkit-box-direction: normal;
+	-webkit-box-orient: vertical;
+
+	-webkit-flex-direction: column;
+	   -moz-flex-direction: column;
+	    -ms-flex-direction: column;
+	        flex-direction: column;
+
+	-webkit-box-align: center;
+	   -ms-flex-align: center;
+	-webkit-align-items: center;
+	   -moz-align-items: center;
+	        align-items: center;
+
+	-webkit-box-pack: center;
+	   -ms-flex-pack: center;
+	-webkit-justify-content: center;
+	   -moz-justify-content: center;
+	        justify-content: center;
+}
+
 /* Global transition speed settings */
 .reveal[data-transition-speed="fast"] .slides section {
 	-webkit-transition-duration: 400ms;
@@ -643,24 +686,12 @@ body {
 	        transition-duration: 1200ms;
 }
 
-.reveal .slides>section.stack {
-	padding-top: 0;
-	padding-bottom: 0;
-}
-
 .reveal .slides>section.present,
 .reveal .slides>section>section.present {
-	display: block;
 	z-index: 11;
 	opacity: 1;
 }
 
-.reveal.center,
-.reveal.center .slides,
-.reveal.center .slides section {
-	min-height: auto !important;
-}
-
 /* Don't allow interaction with invisible slides */
 .reveal .slides>section.future,
 .reveal .slides>section>section.future,
@@ -1162,7 +1193,6 @@ body {
 }
 
 .reveal.overview .slides section {
-	height: 600px;
 	overflow: hidden;
 	opacity: 1 !important;
 	visibility: visible !important;

js/reveal.js

@@ -1178,25 +1178,29 @@ var Reveal = (function(){
 
 			for( var i = 0, len = slides.length; i < len; i++ ) {
 				var slide = slides[ i ];
+				var style = window.getComputedStyle( slide );
 
-				// Don't bother updating invisible slides
-				if( slide.style.display === 'none' ) {
+				// Don't update invisible slides
+				if( style.display === 'none' ) {
 					continue;
 				}
-
-				if( config.center || slide.classList.contains( 'center' ) ) {
-					// Vertical stacks are not centred since their section
-					// children will be
-					if( slide.classList.contains( 'stack' ) ) {
-						slide.style.top = 0;
+				// If the display mode is 'block' flexbox is not supported by
+				// the current browser so we fall back on JavaScript centering
+				else if( style.display === 'block' ) {
+					if( config.center || slide.classList.contains( 'center' ) ) {
+						// Vertical stacks are not centred since their section children will be
+						if( slide.classList.contains( 'stack' ) ) {
+							slide.style.top = 0;
+						}
+						else {
+							slide.style.top = Math.max( ( ( slideHeight - getAbsoluteHeight( slide ) ) / 2 ) - slidePadding, 0 ) + 'px';
+							slide.style.height = 'auto';
+						}
 					}
 					else {
-						slide.style.top = Math.max( ( ( slideHeight - getAbsoluteHeight( slide ) ) / 2 ) - slidePadding, 0 ) + 'px';
+						slide.style.top = '';
 					}
 				}
-				else {
-					slide.style.top = '';
-				}
 
 			}
 
@@ -1926,11 +1930,11 @@ var Reveal = (function(){
 
 				// Show the horizontal slide if it's within the view distance
 				if( distanceX < viewDistance ) {
-					horizontalSlide.style.display = 'block';
+					horizontalSlide.classList.add( 'visible' );
 					loadSlide( horizontalSlide );
 				}
 				else {
-					horizontalSlide.style.display = 'none';
+					horizontalSlide.classList.remove( 'visible' );
 				}
 
 				if( verticalSlidesLength ) {
@@ -1943,11 +1947,11 @@ var Reveal = (function(){
 						distanceY = x === indexh ? Math.abs( indexv - y ) : Math.abs( y - oy );
 
 						if( distanceX + distanceY < viewDistance ) {
-							verticalSlide.style.display = 'block';
+							verticalSlide.classList.add( 'visible' );
 							loadSlide( verticalSlide );
 						}
 						else {
-							verticalSlide.style.display = 'none';
+							verticalSlide.classList.remove( 'visible' );
 						}
 					}