[css-color-5] port w3c#445 here

Author: svgeesus

css-color-5/Overview.bs

@@ -766,6 +766,14 @@ This provides Web compatibility; legacy sRGB content interpolates in the sRGB sp
 Interpolating to or from ''<color>/currentcolor'' is possible.
 The numerical value used for this purpose is the used value.
 
+ISSUE: Computed value needs to be able to represent
+combinations of ''currentColor'' and an actual color.
+Consider the value of 'text-emphasis-color' in
+<code>div { text-emphasis: circle; transition: all 2s; }<br>
+div:hover { text-emphasis-color: lime; }<br>
+em { color: red; }</code>
+See <a href="https://github.com/w3c/csswg-drafts/issues/445">Issue 445</a>.
+
 Color space for interpolation {#interpolation-space}
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