src/language-css/loc.js/getValueRootOffset
function getValueRootOffset(node) {
  let result = node.source.startOffset;
  if (typeof node.prop === "string") {
    result += node.prop.length;
  }

  if (node.type === "css-atrule" && typeof node.name === "string") {
    result +=
      1 + node.name.length + node.raws.afterName.match(/^\s*:?\s*/)[0].length;
  }

  if (
    node.type !== "css-atrule" &&
    node.raws &&
    typeof node.raws.between === "string"
  ) {
    result += node.raws.between.length;
  }

  return result;
}