diff --git a/lib/style-ast.js b/lib/style-ast.js
index d8fc3b8..bb3f401 100644
--- a/lib/style-ast.js
+++ b/lib/style-ast.js
@@ -6,6 +6,7 @@ const stringPattern = /^(["']).*\1$/;
 const maxBase64Length = 1000;
 const excludedSelectors = [ /::?(?:-moz-)?selection/ ];
 const excludedProperties = [
+	/(.*)animation/,
 	/(.*)transition(.*)/,
 	/cursor/,
 	/pointer-events/,
@@ -70,6 +71,7 @@ class StyleAST {
 		);
 
 		clone.pruneMediaQueries();
+		clone.pruneKeyframes();
 		clone.pruneNonCriticalSelectors( criticalSelectors );
 		clone.pruneExcludedProperties();
 		clone.pruneLargeBase64Embeds();
@@ -265,6 +267,21 @@ class StyleAST {
 		} );
 	}
 
+	/**
+	 * Remove keyframe definitions.
+	 */
+	pruneKeyframes() {
+		csstree.walk( this.ast, {
+			visit: 'Atrule',
+			enter: ( atrule, atitem, atlist ) => {
+				// Ignore non-keyframes.
+				if ( csstree.keyword( atrule.name ).basename === 'keyframes' ) {
+					atlist.remove( atitem );
+				}
+			},
+		} );
+	}
+
 	/**
 	 * Returns true if the given CSS rule object relates to animation keyframes.
 	 *