Index: src/wp-content/themes/twentyseventeen/template-parts/post/content-excerpt.php
===================================================================
--- src/wp-content/themes/twentyseventeen/template-parts/post/content-excerpt.php	(revision 40334)
+++ src/wp-content/themes/twentyseventeen/template-parts/post/content-excerpt.php	(working copy)
@@ -30,7 +30,13 @@
 			</div><!-- .entry-meta -->
 		<?php endif; ?>
 
-		<?php the_title( sprintf( '<h2 class="entry-title"><a href="%s" rel="bookmark">', esc_url( get_permalink() ) ), '</a></h2>' ); ?>
+		<?php if ( is_front_page() && ! is_home() ) {
+
+			// The excerpt is being displayed within a front page section, so it's a lower hierarchy than h2.
+			the_title( sprintf( '<h3 class="entry-title"><a href="%s" rel="bookmark">', esc_url( get_permalink() ) ), '</a></h3>' );
+		} else {
+			the_title( sprintf( '<h2 class="entry-title"><a href="%s" rel="bookmark">', esc_url( get_permalink() ) ), '</a></h2>' );
+		} ?>
 	</header><!-- .entry-header -->
 
 	<div class="entry-summary">