Usability and test infrastructure updates.

src/index.css

@@ -16,6 +16,18 @@ body {
   min-height: 100vh;
 }
 
+.visually-hidden {
+  position: absolute;
+  width: 1px;
+  height: 1px;
+  padding: 0;
+  margin: -1px;
+  overflow: hidden;
+  clip: rect(0, 0, 0, 0);
+  white-space: nowrap;
+  border: 0;
+}
+
 .app {
   max-width: 720px;
   margin: 0 auto;