• CSS Box Model: https://developer.mozilla.org/en-US/docs/Learn/CSS/Introduction_to_CSS/Box_model
• box-sizing : https://developer.mozilla.org/en-US/docs/Web/CSS/box-sizing
• More on height & width: https://www.w3schools.com/css/css_dimension.asp
• The display  Property: https://developer.mozilla.org/en-US/docs/Web/CSS/display
• Pseudo Classes on the MDN: https://developer.mozilla.org/en-US/docs/Web/CSS/Pseudo-classes
• Dive deeper into Pseudo Elements: https://developer.mozilla.org/en-US/docs/Web/CSS/Pseudo-elements