<br /> is the same as <br></br>.

writing <!>
is the same as writing <!-- -->

<!DOCTYPE html> is the same as <!--DOCTYPE html-->

Everything in web browser is the same as flowlayout so for iframes
you need a <br /> between them or they will be right next to each other.
eg
<iframe></frame>
<iframe></frame> will be next to each other.
To have them on top of one another you need to do
<iframe></iframe>
<br />
<iframe></iframe>
However <div></div> does <br /> automatically
so you don't have type <br /> for </div>.