[css-align-3] Specify that if a box has no alignment baseline, one is synthesized according to the rules of its formatting context. #4675

Author: fantasai

css-align-3/Overview.bs

@@ -2092,11 +2092,14 @@ Determining the Baselines of a Box</h2>
 
 	Each box,
 	for a given axis,
-	has a <dfn export lt="first baseline set|first baselines">first baseline set</dfn>
+	has potentially
+	a <dfn export lt="first baseline set|first baselines">first baseline set</dfn>
 	(and <dfn export lt="last baseline set|last baselines">last baseline set</dfn>)
 	that nominally corresponds to the <a>baseline set</a> of
 	the first/last line of text within the box.
-	The <dfn export>alignment baseline</dfn> is one of these,
+	The <dfn export>alignment baseline</dfn>,
+	which is the baseline used to align the box in its [=alignment context=],
+	is one of the baselines in its [=baseline set=],
 	usually the dominant baseline associated with the <a>shared alignment context</a>.
 	(See the 'dominant-baseline' and 'alignment-baseline' properties in [[CSS-INLINE-3]].)
 
@@ -2170,6 +2173,10 @@ Determining the Baselines of a Box</h2>
 	use the baseline table from the font settings and <a>first available font</a> of that box,
 	and align that baseline set to the given single baseline.
 
+	If a box that [=participates in baseline alignment=]
+	has no [=baseline set=],
+	then its [=alignment baseline=] is [=synthesized=]
+	according to the rules of the [=formatting context=] in which it participates.
 	To <dfn lt="synthesize baseline|synthesized baseline" local-lt=synthesize|synthesized export>synthesize baselines</dfn> from a rectangle (or two parallel lines),
 	synthesize the alphabetic baseline from the <a>line-under</a> line,
 	and the central baseline by averaging the positions of the two edges or lines.