[css-align-3] Better definition of how baseline self-alignment and content-alignment interact. w3c#6389

Author: fantasai

css-align-3/Overview.bs

@@ -647,9 +647,16 @@ Baseline Alignment: the ''baseline'' keyword and ''first''/''last'' modifiers</h
 	which may affect the sizing of its container.
 	See [[#baseline-align-self]].
 
-	If both <a>baseline content-alignment</a> <em>and</em> <a>baseline self-alignment</a> are specified in the same axis on the same box,
-	only <a>baseline self-alignment</a> is honored in that axis;
-	the content-alignment in that axis must be treated as ''start''.
+	When [=baseline self-alignment=] is specified on a box,
+	[=content distribution=] is performed first,
+	then the resulting box and its contents
+	are [=baseline self-alignment|self-aligned=]
+	However, if the box also has [=baseline content-alignment=] in the same axis
+	or if its [=content-distribution property=] in the same axis is ''normal'',
+	its [=used value|used=] [=content-distribution property=] in that axis
+	is ''align-content/start'' or ''safe end''
+	for a [=baseline-alignment preference=] of its [=baseline self-alignment=]
+	of “first” or “last”, respectively.
 
 	<p class='issue'>Add example images here.