[Resolved] 2 css coding questions, please

I installed the Simple CSS plugin.

I would like to have a simple code for centering youtube videos. I’m wondering if this would work?

iframe { display: block;
margin: auto;
}

If I use that and put it in simple css – I won’t need any extra html when I embed videos?

Question #2
On W3schools they suggest adding this to keep images from being pulled out of shape in a responsive design – is it necessary?

img { max-width: 100%;
height: auto;
}