Adjust how max line width is set on lists and headings.

assets/sass/base/grouping/_grouping.scss

@@ -25,11 +25,6 @@ ul,
 menu {
   // Add vertical rhythm margins.
   @include margin-block(1);
-
-  // Set max text line width in main for readability.
-  .main & {
-    max-width: var(--max-line-width);
-  }
 }
 
 dd {
@@ -57,6 +52,14 @@ menu {
   }
 }
 
+li {
+  // Set max text line width in main for readability.
+  .main & {
+    max-width: var(--max-line-width);
+  }
+}
+
+
 // Figures
 
 figure {

assets/sass/base/headings/_headings.scss

@@ -20,6 +20,11 @@ h6,
   font-family: var(--ff-headings);
   font-weight: var(--fw-headings);
   line-height: var(--heading-line-height);
+
+  // Set max text line width in main for readability.
+  .main & {
+    max-width: var(--max-line-width);
+  }
 }
 
 h1,
@@ -51,19 +56,3 @@ h6,
 %h6 {
   font-size: var(--fs-xs);
 }
-
-h2,
-%h2,
-h3,
-%h3,
-h4,
-%h4,
-h5,
-%h5,
-h6,
-%h6 {
-  // Set max text line width in main for readability.
-  .main & {
-    max-width: var(--max-line-width);
-  }
-}