Âwhen the footer is the last element in a page body, the pseudo-element is used to add a placeholder element after the footer.

The pseudo-element is used to style parts of an element. style the line, style the first letter, insert content before, insert content after . When the footer is the last element in a page body, the after  pseudo-element is used to add a placeholder element after the footer.
The after pseudo element creates a pseudo-element that is the last child of the selected element.

Well, this code would box the whole body of an html document. by using the relative value for position, you are telling it to position relative to its normal position, which the top left corner is at (0,0). by using left: 50px, you are telling the box to set the left margin  edge to 50px and by using top: 50px, you are telling it to set the top margin edge to 50px. by doing this shifting, the top left corner of the box, which is at (0,0) would be positioned at (50,50)

Answerall but the first dimension;