DOM规范中MutationObserver接口观察字符数据无效的疑问

let observer = new MutationObserver((mutationRecords) => console.log(mutationRecords));
document.body.firstChild.textContent = "foo";
observer.observe(document.body.firstChild, {characterData: true});
document.body.firstChild.textContent = "foo";
document.body.firstChild.textContent = "bar";
document.body.firstChild.textContent = "baz";

在firefox Chrome浏览器里试过均无mutationRecords记录，什么原因？