Merge pull request #24 from stephenmcgruer/horizontal_vertical_directions

Specify which ScrollDirections obey writing-mode/directionality

Author: birtles

Overview.bs

@@ -315,24 +315,31 @@ scrollable element.
 
 :   <code>auto</code>
 ::  If only one direction is scrollable, selects that direction.
-    Otherwise selects the direction along the [=block axis=].
+    Otherwise selects the direction along the [=block axis=], conforming to
+    writing mode and directionality.
 
 :   <code>block</code>
-::  Selects the direction along the [=block axis=].
+::  Selects the direction along the [=block axis=], conforming to writing mode
+    and directionality.
 
 :   <code>inline</code>
-::  Selects the direction along the [=inline axis=].
+::  Selects the direction along the [=inline axis=], confirming to writing mode
+    and directionality.
 
 :   <code>horizontal</code>
-::  Selects the horizontal direction.
+::  Selects the physical horizontal direction (ignoring writing mode and
+    directionality).
 
 :   <code>vertical</code>
-::  Selects the vertical direction.
+::  Selects the physical vertical direction (ignoring writing mode and
+    directionality).
 
-<div class="issue">
+Note: Having both logical (block/inline) and physical (vertical/horizontal)
+directions allows web developers to animate both logical (e.g.
+margin-inline-start) and physical (e.g. transform) properties with good
+behavior under different directionalities and writing modes.
 
-Should the physical directions ("horizontal" and "vertical") be removed, leaving 
-only the logical directions ("block" and "inline")?
+<div class="issue">
 
 What about a value that means, "the longest scroll direction"? That would be
 more reliable than "auto" for the case where layout differences could mean that,