Include clarifying comment

scripts/link-checker.js

@@ -227,6 +227,15 @@ async function checkLinks() {
 
  let matchesHash = true;
  if (hash) {
+ // On some websites, the ids may not exactly match the hash included
+ // in the link.
+ // For e.g. GitHub will prepend client facing ids with their own
+ // calculated value. A heading in a README for example could be
+ // Foo bar, navigated to with https://github.com/foo/bar#foo-bar,
+ // but GitHub calculates the actual markup id included in the document
+ // as being user-content-foo-bar for its own page processing purposes.
+ //
+ // See https://github.com/w3c/aria-practices/issues/2809
  matchesHash = !!matchingPage?.ids.some((id) => id.includes(hash));
  }