diff --git a/source/css/_common/outline/sidebar/sidebar-toc.styl b/source/css/_common/outline/sidebar/sidebar-toc.styl
index efeca29..b926524 100644
--- a/source/css/_common/outline/sidebar/sidebar-toc.styl
+++ b/source/css/_common/outline/sidebar/sidebar-toc.styl
@@ -43,6 +43,7 @@ if (hexo-config('toc.enable')) {
 
         .active > .nav-child {
           height: auto;
+          // https://caniuse.com/mdn-css_types_calc-size
           height: calc-size(auto, size);
           opacity: 1;
         }