Element.innerHTML

<div class="greeting">
	<p>Hello world!</p>
</div>

let greeting = document.querySelector('.greeting');

// Get HTML content
// returns "<p>Hello world!</p>"
let html = greeting.innerHTML;

// Set HTML content
// This replaces what was in there already
greeting.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
greeting.innerHTML += ' Add this after what is already there.';

// Add HTML to the beginning of an element's existing content
greeting.innerHTML = 'We can add this to the beginning. ' + elem.innerHTML;

// You can inject entire elements into other ones, too
greeting.innerHTML += '<p>A new paragraph</p>';