Say you wanted to put the CSS-Tricks website in an
<iframe></iframe>. You’d do that like this:
Without any other styling, you’d get a rectangle that is 300×150 pixels in size. That’s not even in the User Agent stylesheet, it’s just some magical thing about iframes (and objects). That’s almost certainly not what you want, so you’ll often see width and height attributes right on the iframe itself (YouTube does this).
<iframe width="560" height="315" src="https://css-tricks.com"></iframe>
Those attributes are … Read article “Responsive Iframes”