why setting the radius of the
#banner .content h1 border to 100% gives me an oval and not a circle?
Having 100% on
border-radius should give you a perfect circle, that is correct, only if the element’s height and width are the same. That is, you need a square element to have a circle border. As well, if your element does not have specific measurements it is at the mercy of the size of the container and its own content, and therefore you will need to adjust its measurements if your goal is to have a circle border.