refactor: current sources

changes:
- `<strong>` → `<h3>`
- add a `.strong` class to mimic the `<strong>` behaviour
- wrap lists in `<ul>`
- provide feedback when no sources are available

public/assets/css/global/typography.css

@@ -67,6 +67,11 @@ h3 {
   font-weight: 500;
 }
 
+h3.strong {
+  color: var(--secondary);
+  font-weight: bolder;
+}
+
 @media only screen and (max-width: 800px) {
   h1 {
     font-size: 2em;