<div class="button-group" focusgroup="menubar">
  <button value="-.9" command="--scroll" commandfor="scroller">Prev</button>
  <button value=".9" command="--scroll" commandfor="scroller">Next</button>
</div>
<ul id="scroller">
  <li></li>
  ...
  <li></li>
</ul>

const scroller = document.querySelector("#scroller");
scroller.addEventListener("command", (e) => {
  if (e.command === "--scroll") {
    scroller.scrollBy({
      left: scroller.offsetWidth * e.source.value
    })
  }
});