[css-inline-3] Improve intro.

Author: fantasai

css-inline-3/Overview.bs

@@ -37,9 +37,16 @@ spec:css-break-3; type:dfn; text:fragment
 <h2 id="intro">
 Introduction</h2>
 
-	This module defines the CSS Inline Layout model,
-	replacing and extending the model as defined in CSS2.1.
-	It is very much a work-in-progress, and implementers should reference CSS2.1 for now.
+	This module defines [=inline layout=], the CSS model
+	for laying out a mixed, continuous stream of text and [=inline-level=] boxes
+	as a stack of lines that form a “paragraph”,
+	and defines controls for the [=block-axis=] alignment and sizing
+	of this content within each line.
+	It also adds a [[#initial-letter-styling|special layout mode for drop caps and similar initial letter styling]].
+
+	Note: Line-breaking, justification, and other aspects of
+	inline-axis positioning of [=inline-level=] content
+	are handled in the <a href="https://www.w3.org/TR/css-text/">CSS Text Module</a>.
 
 	ISSUE: Many aspects of layout here depend on font metrics.
 	While the relevant metrics exist in OpenType for Latin/Cyrillic/Greek and for CJK,
@@ -50,6 +57,13 @@ Introduction</h2>
 	and that means both that OpenType needs to allow such metrics
 	and font designers need to provide accurate numbers.
 
+<h3 id="placement">
+Module Interactions</h3>
+
+  <p>This module
+  replaces and extends the CSS inline layout model and features defined in
+  [[!CSS2]] section 10.8.
+
 <h3 id="values">
 Value Definitions</h3>
 
@@ -66,10 +80,20 @@ Value Definitions</h3>
 <h2 id="model">
 Inline Layout Box Model</h2>
 
+	In <dfn>inline layout</dfn>,
+	a mixed stream of text and [=inline-level boxes=] are laid out
+	by [=fragmenting=] them into a stack of [=line boxes=],
+	and aligning the text and boxes to each other within each [=line box=].
+
 	A [=block container=] element that directly contains
 	[=inline-level=] content--
 	such as [=inline boxes=], [=atomic inlines=], and [=text runs=]--
-	establishes an [=inline formatting context=].
+	establishes an [=inline formatting context=]
+	to lay out its contents.
+	The [=block container=]’s [=content edges=] form the [=containing block=]
+	for each of the [=inline-level boxes=]
+	participating in its [=inline formatting context=].
+
 	The [=block container=] also generates
 	a <dfn export>root inline box</dfn>,
 	which is an <a lt="anonymous box">anonymous</a> [=inline box=] that holds