[css-shadow-parts-1] Expand motivation, remove "for obvious reasons. Fixes #2099

Author: fergald

css-shadow-parts-1/Overview.bs

@@ -50,12 +50,16 @@ without surrending all control.
 Motivation {#motivation}
 ------------------------
 
-For obvious reasons,
-it's valuable to let the outside page style the internals of a shadow tree,
-at least in some limited ways.
-(The ubiquity of UA-specific pseudo-elements for the various input elements shows this.)
-
-The previous proposed method for doing so,
+For custom elements to be fully useful and as capable as built-in elements
+it should be possible for parts of them to be styled from outside.
+Exactly what can be styled from outside should be controlled by the element author.
+Also, it should be possible for a custom element to present a stable "API" for styling.
+That is, the selector used to style a part of a custom element
+should not expose or require knowledge of the internal details of the element.
+The custom element author should be able to change the internal details of the element
+while leaving the selectors untouched.
+
+The previous proposed method for styling inside the shadow tree,
 the >>> combinator,
 turned out to be <em>too powerful</em> for its own good;
 it exposed too much of a component's internal structure to scrutiny,