Attempt to keep anchor targets below the fixed header

Largely using the CSS technique from https://stackoverflow.com/a/28824157/1028376

agaric/sass/agaric/_component.navbar.scss

@@ -28,6 +28,26 @@
   }
 }
 
+// Attempt to keep anchor targets below the fixed header rather than having its
+// associated content hidden behind the header; hence the height and negative
+// margin here match the fixed header height at different breakpoints.
+:target::before {
+  content: "";
+  display: block;
+  height: 3.3125em;
+  margin: -3.3125em 0 0;
+  @include desktop {
+    height: 5.875em;
+    margin: -5.875em 0 0;
+  }
+}
+
+a:target,
+a:target:hover {
+  padding: 0;
+  margin: 0;
+}
+
 .navbar {
   color: $white;
   background-color: $dark;