Note: using innerHTML with third-party or user-submitted content can expose you to cross-site scripting (XSS) attacks.
var elem = document.querySelector('#some-elem');
// Get HTML content
var html = elem.innerHTML;
// Set HTML content
elem.innerHTML = 'We can dynamically change the HTML. We can even include HTML elements like <a href="#">this link</a>.';
// Add HTML to the end of an element's existing content
elem.innerHTML += ' Add this after what is already there.';
// Add HTML to the beginning of an element's existing content
elem.innerHTML = 'We can add this to the beginning. ' + elem.innerHTML;
// You can inject entire elements into other ones, too
elem.innerHTML += '<p>A new paragraph</p>';