Replace heading elements of panel with span.

Fix the heading levels audit.

assets/css/_addon/main.scss

@@ -516,20 +516,15 @@ footer {
   &.topbar-down {
     top: 6rem;
   }
-  >div {
+  > div {
     padding-left: 1rem;
     border-left: 1px solid var(--main-border-color);
     &:not(:last-child) {
       margin-bottom: 4rem;
     }
   }
-  h3 {
-    padding-top: .5rem;
-    padding-bottom: .5rem;
-    margin-top: 0;
-    margin-bottom: 0;
-    letter-spacing: -0.02em;
-    @include label(inherit, 600);
+  span {
+    @include panel-label;
   }
   .post-content {
     font-size: .9rem;

assets/css/_addon/module.scss

@@ -99,6 +99,17 @@
   font-family: 'Roboto Condensed', 'Microsoft Yahei', sans-serif;
 }
 
+@mixin panel-label {
+  display: block;
+  line-height: 1.2;
+  padding-top: .5rem;
+  padding-bottom: .5rem;
+  margin-top: 0;
+  margin-bottom: 0;
+  letter-spacing: -0.02em;
+  @include label(inherit);
+}
+
 @mixin align-center {
   position: relative;
   left: 50%;

assets/css/post.scss

@@ -132,7 +132,6 @@ $prompt-newer: "{{ site.data.label.post.button.next }}";
 }
 
 #toc-wrapper {
-  margin-top: 7rem;
   border-left: 1px solid rgba(158, 158, 158, 0.17);
   position: -webkit-sticky;
   position: sticky;
@@ -142,8 +141,8 @@ $prompt-newer: "{{ site.data.label.post.button.next }}";
   &.topbar-down {
     top: 6rem;
   }
-  >h3 {
-    @include label(inherit, 600);
+  > span {
+    @include panel-label;
   }
 }