Index: src/wp-content/themes/twentysixteen/css/editor-blocks.css
===================================================================
--- src/wp-content/themes/twentysixteen/css/editor-blocks.css	(revision 57538)
+++ src/wp-content/themes/twentysixteen/css/editor-blocks.css	(working copy)
@@ -634,6 +634,13 @@
 	margin-bottom: 16px;
 }
 
+/* Avatar & Post Author */
+
+.editor-styles-wrapper .avatar,
+.wp-block-post-author__avatar img {
+	border-radius: 50%;
+}
+
 /*--------------------------------------------------------------
 7.0 Blocks - Colors
 --------------------------------------------------------------*/