[css-text-3] Clarify segments breaks and interaction with DOM/HTML

See #5147

Author: frivoal

css-text-3/Overview.bs

@@ -1280,28 +1280,48 @@ Order of Operations</h4>
 <h2 id="white-space-processing">
   White Space Processing Details</h2>
 
-  <p>The source text of a document often contains formatting
+    The source text of a document often contains formatting
     that is not relevant to the final rendering: for example,
     <a href="http://rhodesmill.org/brandon/2012/one-sentence-per-line/">breaking the source into segments</a>
     (lines) for ease of editing
     or adding [=white space characters=] such as [=tabs=] and [=spaces=] to indent the source code.
     CSS white space processing allows the author to control interpretation of such formatting:
     to preserve or collapse it away when rendering the document.
-    White space processing in CSS interprets [=white space characters=] only for rendering:
+    White space processing in CSS
+    (which is controlled with the 'white-space' property)
+    interprets [=white space characters=] only for rendering:
     it has no effect on the underlying document data.
 
-  <p>White space processing in CSS is controlled with the 'white-space' property.
+    Note: Depending on the document language,
+    segments can be separated by a particular newline sequence
+    (such as a line feed or CRLF pair),
+    or delimited by some other mechanism,
+    such as the SGML <code>RECORD-START</code> and <code>RECORD-END</code> tokens.
 
-  <p id="segment-normalization">
-    CSS does not define document segmentation rules. Segments can be
-    separated by a particular newline sequence (such as a line feed or
-    CRLF pair), or delimited by some other mechanism, such as the SGML
-    <code>RECORD-START</code> and <code>RECORD-END</code> tokens.
-    For CSS processing, each document language&ndash;defined segment break
-    and each line feed (U+000A)
+    <p id="segment-normalization">
+    For CSS processing, each document language&ndash;defined “segment break” or “newline sequence”--
+    or if none are defined,  each line feed (U+000A)--
     in the text is treated as a <dfn export>segment break</dfn>,
     which is then interpreted for rendering as specified by the 'white-space' property.
 
+    In the case of [[HTML]],
+    each <a href="https://html.spec.whatwg.org/#newlines">newline sequence is normalized</a> to a single line feed (U+000A)
+    for representation in the DOM,
+    so when an HTML document is represented as a [[DOM]] tree
+    each line feed (U+000A)
+    is treated as as a [=segment break=].
+
+    Note: In most common CSS implementations,
+    HTML does not get styled directly.
+    Instead, it is processed into a [[DOM]] tree,
+    which is then styled.
+    Unlike HTML,
+    the DOM does not give any particular meaning to carriage returns (U+000D),
+    so they are not treated as [=segment breaks=].
+    If carriage returns (U+000D) are inserted into the DOM
+    by means other than HTML parsing,
+    they then get treated as defined below.
+
     <wpt pathprefix="/css/CSS2/text">
         white-space-processing-005.xht
         white-space-processing-006.xht