[css-color-5] allow mix percent to default to 50%

Author: svgeesus

css-color-5/Overview.bs

@@ -85,6 +85,9 @@ Issue(4747): <a href="https://github.com/w3c/csswg-drafts/issues/4747">color-fun
 When a plain number or percentage is specified,
 it applies to all color channels.
 
+If the plain number or percentage is omitted,
+it defaults to 50% (an equal mix of the two colors).
+
 <div class="example">
 	This example produces a mixture of 40% <span class="swatch" style="--color: peru"></span> peru
 	and 60% <span class="swatch" style="--color: lightgoldenrod"></span> lightgoldenrod.