[css-inline-3] Add subheadings (like in the Flexbox algo)

Author: fantasai

css-inline-3/Overview.bs

@@ -207,14 +207,16 @@ Layout Within Line Boxes</h3>
 	sizing and positioning each [=box fragment=] and [=line box=] independently,
 	as follows:
 
-	1. All [=in-flow=] [=inline-level boxes=] in the [=line box=]
+	1. <strong>Baseline Alignment:</strong>
+		All [=in-flow=] [=inline-level boxes=] in the [=line box=]
 		are aligned to each other in the [=block axis=]
 		according to 'dominant-baseline' and 'vertical-align'.
 		This is referred to as [[#alignment|baseline alignment]].
 		Those with [=line-relative values=] for 'baseline-shift'
 		are assumed to be aligned so as to minimize the line box height.
 
-	2. The [=layout bounds=] (i.e. the size contributions)
+	2. <strong>Content Size Contribution Calculation:</strong>
+		The [=layout bounds=] (i.e. the size contributions)
 		of each [=inline-level box=] in the [=line box=]
 		are calculated:
 
@@ -236,11 +238,13 @@ Layout Within Line Boxes</h3>
 				see [[#inline-height]]
 		</ul>
 
-	3. The [=line box=]’s [=logical height=] is sized
+	3. <strong>Line Box Sizing:</strong>
+		The [=line box=]’s [=logical height=] is sized
 		to exactly include the aligned [=layout bounds=]
 		of all its [=inline-level boxes=].
 
-	4. The [=root inline box=]’s [=aligned subtree=]
+	4. <strong>Content Positioning:</strong>
+		The [=root inline box=]’s [=aligned subtree=]
 		and boxes [=line-relative values=] for 'baseline-shift'
 		are positioned within the [=line box=].