New default value for the `background-repeat` property

[sandren]

I think the historical default value of the background-repeat property is a perfect example of what the CSSWG would set differently if UA stylesheets were being written today. While repeating background patterns may have been common in the early web, they are now the exception, not the norm. As such I think we should consider changing the default behavior for background repetition.

*, ::before, ::after {
  background-repeat: no-repeat;
  box-sizing: border-box;
}