Fix incorect default values for background CSS shorthand property page.

[vituluss]

Fixed incorrect default values.

The default values for background-clip and background-origin were the wrong way around.

Description

I swapped the written default values for background-clip and background-origin the correct way around as per the specification. The CSS spec says that background resets unspecified properties to initial.

Motivation

MDN docs should have correct information.

Additional details

This is what the CSS spec says:

Given a valid declaration, for each layer the shorthand first sets the corresponding value of each of background-image, background-position, background-size, background-repeat, background-origin, background-clip and background-attachment to that property’s initial value, then assigns any explicit values specified for this layer in the declaration