diff --git a/testsuite/meson.build b/testsuite/meson.build
index 3102a485f8..7c671c4c08 100644
--- a/testsuite/meson.build
+++ b/testsuite/meson.build
@@ -10,6 +10,7 @@ common_env = [
   'G_ENABLE_DIAGNOSTIC=0',
   'GSETTINGS_SCHEMA_DIR=@0@'.format(gtk_schema_build_dir),
   'GDK_DEBUG=default-settings',
+  'GTK_DEBUG=css',
 ]
 exclude_unstable = ['flaky', 'failing']