JSFiddle - React, Tailwind, and code Playground

HTML

<iframe id="one">
</iframe>

<iframe id="two">
</iframe>

JavaScript

window.onload = async function () {
    const iframes = document.querySelectorAll("iframe");
    
    for (let iframe of iframes) {
        await new Promise(resolve => {
            iframe.onload = resolve;
            iframe.src = "about:blank";
        });
        
        iframe.focus();
        const focused = iframe.contentDocument.hasFocus();
        console.assert(focused, "The iframe is not focused.");
    }
};