root scrollbar colors also set on viewport w3c#2696

Per issue w3c#2696, root scrollbar colors also set viewport scrollbar colors. Also add explicit intro paragraph about where spec source is because "Repository:" metadata block appears to do nothing (does not work to add a link to the GitHub repo).

Author: tantek

css-scrollbars-1/Overview.bs

@@ -18,6 +18,11 @@ Link Defaults: css-color-3 (property) color
 <h2 id="intro">
 Introduction</h2>
 
+Note: this specification is 
+<a href="https://github.com/w3c/csswg-drafts/tree/master/css-scrollbars-1">
+maintained in the W3C csswg-drafts GitHub repository as css-scrollbars-1
+</a>.
+
 <h3 id="scope">
 Scope</h3>
 <em>This subsection is non-normative.</em>
@@ -69,14 +74,19 @@ Percentages: n/a
 Media: visual
 </pre>
 
-These properties allow the author to set colors for various aspects of a scrollbar.
+These properties allow the author to set colors for various aspects of an element’s scrollbars.
 
 Note: Per <a href="https://github.com/w3c/csswg-drafts/issues/1960">Issue 1960</a>, 
 the list of scrollbar-*-color properties is likely to be reduced to 2-3 (ideally 2), 
 with current candidates being scrollbar-base-color, scrollbar-face-color, scrollbar-track-color, 
 per <a href="https://www.w3.org/wiki/Css-scrollbars#Use-cases">research so far</a>, 
 and preferably only two of those. Additional input, data, suggestions welcome at those links!
 
+UAs must apply the scrollbar-* values set on the root element to the viewport.
+
+Note: Unlike 'overflow' (and overflow-*) properties, 
+scrollbar-* values set on the HTML body element are not propagated to viewport.
+
 <dl dfn-type="value" dfn-for="scrollbar-base-color">
 <dt><dfn>normal</dfn>
 <dd>default platform rendering for that portion of the scrollbar, in the absence of any other