How to pass the entire DOM of the current page to background.js using contentScript? - google-chrome-extension

Question

How do I pass the entire DOM content of Codechef.com to the background.js via contentScript?
I just want to get the problem tag of codechef question. This will help me to fetch the problem code. For this I need to query the dom.
{
"manifest_version": 3,
"name": "CodechefNotifier",
"version": "1.0",
"description": "Notifier for CodeChef",
"action": {
"default_popup": "index.html"
},
"permissions": [
"webRequest", "debugger", "tabs", "<all_urls>", "notifications", "scripting",
"activeTab"
],
"host_permissions": [
"https://www.codechef.com/"
],
"background": {
"service_worker": "background.js"
},
"content_scripts":[
{
"matches":["<all_urls>"],
"js":["contentScript.js"]
}
]
}
const button = document.createElement('button');
button.textContent = 'Greet me!'
document.body.insertAdjacentElement('afterbegin', button);
var tags = [ "h1" ];
var all_headings = [];
var h1s = document.querySelectorAll("h1");
all_headings.push(document.body.innerHTML);
var urlRegex = /^https?:\/\/(?:[^./?#]+\.)?codechef\.com/;
// A function to use as callback
function doStuffWithDom(domContent) {
console.log('I received the following DOM content:\n' + domContent);
}
// When the browser-action button is clicked...w
window.addEventListener('load',function (tab) {
// ...check the URL of the active tab against our pattern and...
if (urlRegex.test(tab.url)) {
// ...if it matches, send a message specifying a callback too
chrome.tabs.sendMessage(tab.id, {text: 'report_back'}, doStuffWithDom);
}
I want to pass dom content to the background script