[css-scrollbars] Improve scope section

Requests for ::webkit-pseudos or something similar to them keep coming, suggesting we need more clarity.

See w3c#2009 w3c#1969 w3c#2153

Author: frivoal

css-scrollbars-1/Overview.bs

@@ -29,9 +29,9 @@ and thickness ('scrollbar-width').
 <h3 id="scope">
 Scope</h3>
 
-The CSS Scrollbars Module is specifically for styling scrollbar controls themselves, 
-e.g. their color &amp; width in Level 1, and not their layout nor whether any content is scrollable. 
-All layout impacts and content scrollability are specified in the 
+The CSS Scrollbars Module is specifically for styling scrollbar controls themselves,
+e.g. their color &amp; width in Level 1, and not their layout nor whether any content is scrollable.
+All layout impacts and content scrollability are specified in the
 <a href="https://drafts.csswg.org/css-overflow/">CSS Overflow Module</a>.
 
 Based on
@@ -40,28 +40,32 @@ there are three main use-cases around scrollbars this module intends to resolve:
 <ol>
  <li>Coloring scrollbars to fit better into the UI of a web application.
  <li>Using a thinner scrollbar when the scrolling area is small.
- <li>Building customized scrollbars without affecting scrollability.
+ <li>Hiding UA-provided scrollbars,
+  to allow the provision of custom interfaces for scrolling
+  without affecting other aspects of scrollability.
 </ol>
 
-For addressing these use-cases, this module adds properties to control the colors and
-width of scrollbars (if any) of an element.
-
 <h4 id="out-of-scope">
 Out Of Scope</h4>
-Layout of scrollbars and related or adjacent content, and whether content is scrollable, is out of scope.
-Those features are specified in the 
-<a href="https://drafts.csswg.org/css-overflow/">CSS Overflow Module</a>.
 
-Pseudo-elements for selecting specific parts of a scrollbar are out of scope.
-The WebKit implementation of pseudo-elements for scrollbar is
-considered to be a feature mistakenly exposed to the web.
-Main concerns against the pseudo-elements approach are:
-<ul>
- <li>Operating systems continuously evolve their scrollbar designs to provide better user experience,
- beyond the ability of any set of pseudo-elements to accurately model this over time.
- <li>Different platforms have different scrollbar structure means testing interop is harder,
- because authors would need to take not only engine but also platforms into account.
-</ul>
+The internal structure, layout, and configuration of scrollbars,
+as well as precise control over their coloring, is out of scope.
+This is because different platforms have
+different scrollbar structures and styling conventions,
+and operating systems continuously evolve their scrollbar designs
+to provide better user experience.
+Pseudo-elements for selecting specific parts of a scrollbar,
+for example,
+were considered and rejected.
+While this level of fine control would be tempting for authors,
+the arrangement of the various parts--
+or whether they're even all present--
+cannot be depended on.
+Providing too much control would allow authors to get perfect results on some platforms,
+but at the expense of broken results on others.
+
+Note: Exposing the scrollbar-related <code>::-webkit-</code> prefixed pseudo-elements to the Web
+is considered a mistake by both the CSS Working Group and Webkit.
 
 <h3 id="values">
 Value Definitions</h3>