z-index and stacking contexts Edit

Created 9/4/2013, updated 5/18/2017

"The stacking context": https://developer.mozilla.org/en-US/docs/Web/Guide/CSS/Understanding_z_index/The_stacking_context?redirectlocale=en-US&redirectslug=CSS%2FUnderstanding_z-index%2FThe_stacking_context
"Appendix E. Elaborate description of Stacking Contexts": http://www.w3.org/TR/CSS2/zindex.html
"What No One Told You About Z-Index": http://philipwalton.com/articles/what-no-one-told-you-about-z-index/