add offline support

test/mobile-navigation-ui.test.js

@@ -16,13 +16,16 @@ async function clientSources() {
   return Promise.all(names.filter(name => name.endsWith('.js')).map(name => read(`public/js/${name}`)));
 }
 
-test('every static HTML id is referenced by authored client JavaScript', async () => {
+test('every static HTML id is referenced by client JavaScript or accessible HTML relationships', async () => {
   const html = await read('public/index.html');
   const scripts = (await clientSources()).join('\n');
   const ids = [...html.matchAll(/\bid="([^"]+)"/g)].map(match => match[1]);
+  const htmlReferences = new Set([...html.matchAll(/\b(?:aria-labelledby|aria-describedby|aria-controls|for)="([^"]+)"/g)]
+    .flatMap(match => match[1].split(/\s+/)));
 
   assert.ok(ids.length > 0);
   for (const id of ids) {
+    if (htmlReferences.has(id)) continue;
     assert.match(scripts, new RegExp(id.replace(/[.*+?^${}()|[\]\\]/g, '\\$&')), `unused id: ${id}`);
   }
 });