(() => {
    let f = () => {
        Array.prototype.slice.call(document.querySelectorAll('*'))
            .filter(_ => !_.childElementCount && _.innerText)
        .forEach(_ =>  _.innerText = 'xxx' + _.innerText);
    o.takeRecords();
    };
    let o = new MutationObserver(f);
    o.observe(document,({ characterData: true, subtree: true }))
    f();
})()