Absolute vs. Relative — Explaining CSS Positioning

 https://www.lifewire.com/absolute-vs-relative-3466208

https://css-tricks.com/absolute-relative-fixed-positioining-how-do-they-differ/

https://css-tricks.com/absolute-positioning-inside-relative-positioning/

https://css-tricks.com/fixed-headers-and-jump-links-the-solution-is-scroll-margin-top/