Index: src/wp-content/themes/twentyseventeen/assets/css/ie9.css
===================================================================
--- src/wp-content/themes/twentyseventeen/assets/css/ie9.css	(nonexistent)
+++ src/wp-content/themes/twentyseventeen/assets/css/ie9.css	(working copy)
@@ -0,0 +1,37 @@
+/*
+Theme Name: Twenty Seventeen
+Description: IE9 specific styles.
+*/
+
+.has-header-image.twentyseventeen-front-page .custom-header,
+.has-header-image.home.blog .custom-header {
+	height: 300px;
+}
+
+.has-header-image .custom-header-image img,
+.has-header-image .custom-header-image video,
+.has-header-image .custom-header-image iframe {
+	min-width: 100%;
+}
+
+@media screen and (min-width: 30em) {
+
+	.has-header-image.twentyseventeen-front-page .custom-header,
+	.has-header-image.home.blog .custom-header,
+	.twentyseventeen-front-page.has-header-image .custom-header-image,
+	.home.blog.has-header-image .custom-header-image,
+	.panel-image {
+		height: 700px;
+	}
+}
+
+@media screen and (min-width: 48em) {
+
+	.has-header-image.twentyseventeen-front-page .custom-header,
+	.has-header-image.home.blog .custom-header,
+	.twentyseventeen-front-page.has-header-image .custom-header-image,
+	.home.blog.has-header-image .custom-header-image,
+	.panel-image {
+		height: 1000px;
+	}
+}
Index: src/wp-content/themes/twentyseventeen/functions.php
===================================================================
--- src/wp-content/themes/twentyseventeen/functions.php	(revision 39163)
+++ src/wp-content/themes/twentyseventeen/functions.php	(working copy)
@@ -356,6 +356,12 @@
 		wp_enqueue_style( 'twentyseventeen-colors-dark', get_theme_file_uri( '/assets/css/colors-dark.css' ), array( 'twentyseventeen-style' ), '1.0' );
 	}
 
+	// Load the Internet Explorer 9 specific stylesheet, to fix display issues in the Customizer.
+	if ( is_customize_preview() ) {
+		wp_enqueue_style( 'twentyseventeen-ie9', get_theme_file_uri( '/assets/css/ie9.css' ), array( 'twentyseventeen-style' ), '1.0' );
+		wp_style_add_data( 'twentyseventeen-ie9', 'conditional', 'IE 9' );
+	}
+
 	// Load the Internet Explorer 8 specific stylesheet.
 	wp_enqueue_style( 'twentyseventeen-ie8', get_theme_file_uri( '/assets/css/ie8.css' ), array( 'twentyseventeen-style' ), '1.0' );
 	wp_style_add_data( 'twentyseventeen-ie8', 'conditional', 'lt IE 9' );