How to add css and html in iFrame using JavaScript

 var container = document.querySelector('iframe#gorgias-chat-container');

    var chatButtonHead = container?.querySelector('#chat-button')?.contentWindow.document.querySelector('head');

    var chatButton = container?.querySelector('#chat-button')?.contentWindow.document.querySelector('body');

    chatButton = chatButton?.querySelector('button');

    

    if (chatButton?.children.length){

        var chatbuttonStyle = document.createElement('div');

        chatbuttonStyle.style.textAlign = "center";

    chatbuttonStyle.style.fontWeight = "500";

        chatbuttonStyle.textContent = 'Chat';

    chatButton.appendChild(chatbuttonStyle);

        window.clearInterval(gorgiasChatbutton);

    }

Example 2 

var gorgiasChatbutton = window.setInterval(function() {

    var container = document.querySelector('#gorgias-chat-container');

    var chatButtonHead = container?.querySelector('#chat-button')?.contentWindow.document.querySelector('head');

    var chatButton = container?.querySelector('#chat-button')?.contentWindow.document.querySelector('body');

    var chatButton = container?.querySelector('#chat-button')?.contentWindow.document.querySelector('body');

    chatButton = chatButton?.querySelector('button');

    chatButton = chatButton?.querySelector('.messenger-button-iframe-uscp54');

    chatButton = chatButton?.querySelector('.messenger-button-iframe-clht14');

    

    if (chatButton?.children.length){

        var buttonStyle = document.createElement('style');

    buttonStyle.textContent = '.chattext{color: #0e2fb5;text-align: center;font-weight: 500;position: absolute;bottom: 6px;left: 2px;font-size: 12px;background: #FFF;border-radius: 100%;}'; // the custom CSS for the chat button

    

    chatButtonHead.appendChild(buttonStyle);

        

        var chatbuttonStyle = document.createElement('div');

    chatbuttonStyle.classList.add("chattext")

        chatbuttonStyle.textContent = 'Chat';

    chatButton.appendChild(chatbuttonStyle);

    window.clearInterval(gorgiasChatbutton);

    }

}, 100);