Clarify comments regarding overflow in svg.css

third_party/WebKit/Source/core/css/svg.css

 /*
  * The default style sheet used to render SVG.
  *
  * Copyright (C) 2005, 2006 Apple Computer, Inc.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
@@ skipping 21 matching lines @@
     When an outermost SVG 'svg' element is stand-alone or embedded inline within a parent XML grammar 
     which does not use CSS layout [CSS2-LAYOUT] or XSL formatting [XSL], the 'overflow' property on the 
     outermost 'svg' element is ignored for the purposes of visual rendering and the initial clipping path is set 
     to the bounds of the initial viewport.
 
     When an outermost 'svg' element is embedded inline within a parent XML grammar which uses CSS layout
     [CSS2-LAYOUT] or XSL formatting [XSL], if the 'overflow' property has the value hidden or scroll, then
     the user agent will establish an initial clipping path equal to the bounds of the initial viewport; otherwise,
     the initial clipping path is set according to the clipping rules as defined in [CSS2-overflow].
 
-    Opera/Firefox & WebKit agreed on NOT setting "overflow: hidden" for the outermost svg element - SVG 1.1 Errata
+    Opera/Firefox & WebKit agreed on NOT setting "overflow: hidden" for a root svg element - SVG 1.1 Errata
     contains these changes as well as all future SVG specifications: see http://lists.w3.org/Archives/Public/public-svg-wg/2008JulSep/0347.html
+    Note that here "root svg element" means a frame with purely SVG content, and in which SVG is the root element.

[fs, 2017/03/29 21:35:00]

This could also be referred to as "when <svg> is the document element". I.e:

... means when <svg> is the document element. (":root" in CSS parlance.)

[chrishtr, 2017/03/30 00:43:14]

Done.

+    LayoutSVGRoot objects embededed within containing content get overflow: hidden by default.

[fs, 2017/03/29 21:35:00]

Nit: s/embededed/embedded/

[chrishtr, 2017/03/30 00:43:14]

Done.

+    https://svgwg.org/svg2-draft/single-page.html#render-OverflowAndClipProperties
 */
 
 svg:not(:root), symbol, image, marker, pattern, foreignObject {
     overflow: hidden
 }
 
 svg:root {
     width: 100%;
     height: 100%;
 }
@@ skipping 18 matching lines @@
 
 /* CSS transform specification: "transform-origin 0 0 for SVG elements without associated CSS layout box, 50% 50% for all other elements". */
  
 * {
     -webkit-transform-origin: 0 0;
 }
 
 html|* > svg {
     -webkit-transform-origin: 50% 50%;
 }