According to the CSS Color Module Level 4 spec, when using rgba() function, if the 4th parameter(alpha) is omitted, it should default to 100%. Edge and Safari don't currently do this. Tested with Safari Technology Preview 73. Chrome and Firefox handle it correctly. Test: https://codepen.io/TehShrike/pen/MLYajK Spec: https://www.w3.org/TR/css-color-4/#rgb-functions
I fixed this in http://trac.webkit.org/r239574. Please test STP 74 when it comes out.
<rdar://problem/47459951>
Thanks Simon!
Confirmed fixed in STP 74. Thanks again!