-webkit-border-before - CSS: Cascading Style Sheets
it relates to -webkit-border-after, -webkit-border-start, and -webkit-border-end, which define the other borders of the element.

WebKit CSS extensions - CSS: Cascading Style Sheets
a -webkit-animation-trigger -webkit-app-region -webkit-appearance* -webkit-aspect-ratio b -webkit-backdrop-filter -webkit-background-composite -webkit-border-after** -webkit-border-after-color** -webkit-border-after-style** -webkit-border-after-width** -webkit-border-before** -webkit-border-before-color** -webkit-border-before-style** -webkit-border-before-width** -webkit-border-end** -webkit-border-end-color** -webkit-border-end-style** -webkit-border-end-width** -webkit-border-fit -webkit-border-horizontal-spacing -webkit-border-start** -we...