I have read on w3schools.com that there is this default value in every browser:

iframe:focus { outline: none; }

So I tried to change it to

iframe:focus { outline: 10px solid red; }

Here is the example (You have to click the first button and then use tab to navigate to the iframe.):

iframe:focus
{
  outline:10px solid red;
}

input:focus
{
  background:blue;
}

<input type="button" value="button1" />
<input type="button" value="button2" />
<iframe src="about:blank"></iframe>

It seems that an iframe can't get the focus. The same is said in 34157775.

So my question is: Why is there a default value for iframe:focus on w3schools.com if an iframe can't even get the focus? Is this a bug on w3schools.com?