JavaScript Can Change HTML Content

content.js

document.getElementById("demo").innerHTML = "Hello JavaScript!"
document.getElementById('demo').innerHTML = 'Hello JavaScript';
//---------------
document.getElementById("demo").style.fontSize = "35px";
//or
document.getElementById('demo').style.fontSize = '35px';
//-----------
document.getElementById("demo").style.display = "none";
//or
document.getElementById('demo').style.display = 'none';
//----
document.getElementById("demo").style.display = "block";
//or
document.getElementById('demo').style.display = 'block';