[css-inline-3] Define how initial letter is positioned wrt its containing block per <https://lists.w3.org/Archives/Public/www-style/2018Nov/0007.html> #719

Author: fantasai

css-inline-3/Overview.bs

@@ -1907,6 +1907,25 @@ Sizing the Initial Letter Box
 	excluding any that hang
 	(see 'hanging-punctuation')--
 	as well as the margin boxes of any atomic inlines it contains.
+	However, if its [=block-start=] [=padding=] and [=border=] are both zero,
+	then its [=block-start=] [=content edge=] instead coincides
+	with its [=over=] alignment point instead,
+	and any glyph ink above that point is ignored for the purpose of layout.
+
+	Note: If an [=inline initial letter=] has ascenders above its [=over=] alignment point,
+	and the author has not provided sufficient [=margin=]
+	on either the [=initial letter=] itself or its [=containing block=],
+	then those ascenders might collide with preceding content.
+
+	Note: It might be nice to automatically provide the necessary spacing
+	by treating such ascenders as a margin that can collapse
+	with the margin of the containing block,
+	and thus guarantee the requisite spacing without imposing any additional space
+	unless it becomes actually necessary.
+	Depending on implementation complexity,
+	this option may be explored in the future;
+	but in the meantime,
+	authors need to be careful to provide the requisite spacing explicitly.
 
 	ISSUE: Should the hanging punctuation be included in the box instead
 	(so that the box is drawn around the punctuation when it is made visible through borders/background),
@@ -2000,6 +2019,7 @@ Space Below Initial Letters
 Block-axis Positioning
 
 	In the block axis, the initial letter is positioned
+	with respect to the [=line box=] in which it originates
 	as required to satisfy its under alignment point ('initial-letter-align')
 	at its specified [=initial letter sink|sink=] ('initial-letter'),
 	i.e. it is positioned
@@ -2017,7 +2037,13 @@ Block-axis Positioning
 		          is not affected by the content of the spanned lines.">
 	
 
-	Its position is anchored with respect to the line on which it occurs.
+	The [=initial letter=] does not increase the [=logical height=]
+	of the [=line box=] in which it participates:
+	it can protrude above or below it.
+	It must be positioned such that its own [=block-start=] [=margin edge=]
+	is below its [=containing block=]’s [=block-start=] [=content edge=],
+	and thus can force its originating [=line box=] (and subsequent content)
+	to shift further away from that edge.
 
 
 Inline Positioning and Kerning