[css-scrollbar] Put some reasoning into the scope section.

Author: upsuper

css-scrollbars-1/Overview.bs

@@ -27,15 +27,28 @@ maintained in the W3C csswg-drafts GitHub repository as css-scrollbars-1
 Scope</h3>
 <em>This subsection is non-normative.</em>
 
-This module adds color properties for styling the scrollbar (if any) of an element per 
+Based on
 <a href="https://www.w3.org/wiki/Css-scrollbars#Use-cases">documented use-cases</a>,
-and as has been increasingly adopted 
-on the web since Windows IE 5.5 introduced it in 2000.
-[Note: Add citations of examples from Bugzilla 77790 and webcompat].
+there are three main use-cases around scrollbars this module intends to resolve:
+<ol>
+ <li>Color scrollbars to fit better into the UI of a web application.
+ <li>Use thinner scrollbar when the scrolling area is small.
+ <li>Build customized scrollbars without affecting scrollability.
+</ol>
 
-Out of scope: 
-<a href="https://www.w3.org/wiki/Css-scrollbars#Why_not_pseudos">pseudo-elements for selecting specific parts of a scrollbar are unnecesssary</a>
-for the documented use-cases and are thus out of scope for this specification.
+For addressing these use-cases, this module adds properties to control colors and
+width of scrollbars (if any) of an element.
+
+Pseudo-elements for selecting specific parts of a scrollbar is 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>
 
 <h3 id="values">
 Values</h3>