JSFiddle - React, Tailwind, and code Playground

HTML

<body>
    <frameset cols="25%,40%,*%">
        <frame src="contentstart.html" name="content" />
        <frame src="content.html" name="content2" />
        <frame src="content.html" name="content3" />
    </frameset>
</body>

JavaScript

window.setInterval(function () {
check();
}, 50);

function check() {

    var content1 = document.getElementsByName("content")[0].contentDocument.body;
    var content3 = document.getElementsByName("content3")[0].contentDocument.body;

    if (isEmpty(content3)) {
        if (isEmpty(content1)) setProperties("0px, *, 0px");
        else setProperties("35%, *, 0px");
    } else setProperties("25%, 40%, 35%");

    window.localStorage.clear();
}

function isEmpty(e) {
    return (e.offsetWidth != 0);
}

function setProperties(value) {
    document.getElementsByTagName("frameset")[1].cols = value;
}