body {
  --cssx-children: container todo-container;
}

#container {
  --cssx-children: counter btn-increment;
  --count: '0';
}

#counter {
}
#counter::before {
  content: 'Count: ' var(--count);
}

#btn-increment {
  display: inline-block;
  padding: 0.5rem 1rem;
  border: 1px solid gray;

  --cssx-on-click: update(container, --count, calc(var(--count) + 1));
}
#btn-increment::after {
  content: 'Increment';
}