1. "Focus ring" (focus indicator) is completely missing for buttons. I cannot navigate the site with my keyboard if I don't see the currently focused element. Never remove the outline for focusable elements without providing a sufficient alternative.
2. The focus ring for the form elements is insufficient, especially for input fields.
3. The green, blue and red buttons have insufficient contrast ratio. Use something like [0] to find accessible color combinations.
4. Only native components are used (button, input) - that's great! This way, almost no ARIA is needed. Adding role="alert" [1] to the balloons is the only one I'm missing.
5. The mobile layout is broken for inputs.
Unfortunately, many CSS frameworks either partially or fully ignore the accessibility aspect. It takes a lot of experience and time to make a website fully WCAG-compliant, but it's quite easy to follow some basic guidelines, like in this cheatsheet [2].
[0] https://contrast-ratio.com/