What does the CSS display property control, and how do block, inline, and inline-block differ in three concrete ways?

What is the difference between position: relative and position: absolute? Show a parent/child pair where the child is pinned to the top-right corner of the parent.

Build a sticky footer that always sits at the bottom of the viewport when the content is short, but is pushed down by long content. Use flexbox.

Two elements both have z-index: 9999 but one still appears under the other. What is going on? Name two things that create a new stacking context.