standardize HTML5 element formatting

I am not sure about this change, but wanted to suggest it anyway. I made a page where I did not use Contao’s standard CSS framework from layout.css. So I skimmed through layout.css to check for any goodies I would be missing. I came across [these lines](https://github.com/contao/core/blob/master/assets/contao/css/layout-uncompressed.css#L21-L23) and thought they should actually be included in reset.css as well. A lot of the other normalizations performed by layout.css are already part of reset.css, but this one is not.