remove special "alpha hex value" entry (#4245)
remove special "alpha hex value" entry (#4245)

This information already exists at https://developer.mozilla.org/en-US/docs/Web/CSS/color, and the copy here is incomplete anyway.
mdn/browser-compat-data,chrisdavidmills/browser-compat-data,mdn/browser-compat-data,chrisdavidmills/browser-compat-data