One.re

let (left, width) =
  Webapi.Dom.(
    switch (Js.toOption(node)) {
    | Some(element) =>
      switch (
        element |> Element.querySelector("[aria-selected=\"true\"]")
      ) {
      | Some(element) =>
        switch (element |> Element.asHtmlElement) {
        | Some(element) => (
            element |> HtmlElement.offsetLeft,
            element |> HtmlElement.offsetWidth,
          )
        | _ => (0, 0)
        }
      | _ => (0, 0)
      }
    | _ => (0, 0)
    }
  );

Two.re

let (left, width) =
  Belt.Option.(
    Webapi.Dom.(
      Js.toOption(node)
      ->flatMap(Element.querySelector("[aria-selected=\"true\"]"))
      ->flatMap(Element.asHtmlElement)
      ->map(element =>
          (
            element |> HtmlElement.offsetLeft,
            element |> HtmlElement.offsetWidth,
          )
        )
      ->getWithDefault((0, 0))
    )
  );