[cssom-view] Change scrollIntoView(undefined) to be like true

This aligns with the behavior of Gecko/WebKit/Chromium; EdgeHTML
treats explicit undefined like false.

Fixes #1367.

Tests: web-platform-tests/wpt#6253

Author: zcorpan

cssom-view/Overview.bs

@@ -768,7 +768,7 @@ When asked to <dfn>evaluate media queries and report changes</dfn> for a {{Docum
 <div class='example'>
     A simple piece of code that detects changes in the orientation of the viewport can be written as follows:
 
-    <pre class='lang-javascript'>
+    <pre highlight=javascript>
     function handleOrientationChange(event) {
         if(event.matches) // landscape
             &hellip;
@@ -922,7 +922,7 @@ Note: Some non-conforming implementations are known to return 32 instead of 24.
   <code>colorDepth</code> can be used in the context of selecting SDR/HDR in
   addition with other information. For example, combined with color gamut.
 
-  <pre class='lang-javascript'>
+  <pre highlight=javascript>
   if (screen.colorDepth >= 48   && window.matchMedia('(color-gamut: p3)').matches &&
       /* other checks */) {
     // Use HDR content.
@@ -1082,8 +1082,7 @@ dictionary ScrollIntoViewOptions : ScrollOptions {
 partial interface Element {
   sequence&lt;DOMRect> getClientRects();
   [NewObject] DOMRect getBoundingClientRect();
-  void scrollIntoView();
-  void scrollIntoView((boolean or object) arg);
+  void scrollIntoView(optional (boolean or ScrollIntoViewOptions) arg = true);
   void scroll(optional ScrollToOptions options);
   void scroll(unrestricted double x, unrestricted double y);
   void scrollTo(optional ScrollToOptions options);
@@ -1129,7 +1128,7 @@ Note: The {{DOMRect}} object returned by {{Element/getBoundingClientRect()}} is
 <div class='example'>
     The following snippet gets the dimensions of the first <code>div</code> element in a document:
 
-    <pre class='lang-javascript'>
+    <pre highlight=javascript>
     var example = document.getElementsByTagName("div")[0].getBoundingClientRect();
     var exampleWidth = example.width;
     var exampleHeight = example.height;
@@ -1138,18 +1137,24 @@ Note: The {{DOMRect}} object returned by {{Element/getBoundingClientRect()}} is
 
 The <dfn method for=Element caniuse=scrollintoview>scrollIntoView(<var>arg</var>)</dfn> method must run these steps:
 
-<!-- scrollIntoView() means arg=true but scrollIntoView(undefined) means arg=false, to align with impls. -->
-
 1. Let <var>options</var> be null.
-1. If <var>arg</var> is an object, let <var>options</var> be <var>arg</var>.
-1. <a lt="converted to an IDL value">Convert</a> <var>options</var> to a {{ScrollIntoViewOptions}} dictionary. [[!WEBIDL]]
-1. If <var>arg</var> is not specified or is true, let the {{ScrollIntoViewOptions/block}} dictionary member of <var>options</var> have the value "<code>start</code>", and let the {{ScrollIntoViewOptions/inline}} dictionary member of <var>options</var> have the value "<code>nearest</code>".
-1. If <var>arg</var> is false, let the {{ScrollIntoViewOptions/block}} dictionary member of <var>options</var> have the value "<code>end</code>", and let the {{ScrollIntoViewOptions/inline}} dictionary member of <var>options</var> have the value "<code>nearest</code>".
+1. If <var>arg</var> is a {{ScrollIntoViewOptions}} dictionary, set <var>options</var> to <var>arg</var>. Otherwise:
+    1. <a lt="converted to an IDL value">Convert</a> <var>options</var> to a {{ScrollIntoViewOptions}} dictionary. [[!WEBIDL]]
+    1. If <var>arg</var> is true, set the {{ScrollIntoViewOptions/block}} dictionary member of <var>options</var> to "<code>start</code>", and set the {{ScrollIntoViewOptions/inline}} dictionary member of <var>options</var> to "<code>nearest</code>".
+    1. If <var>arg</var> is false, let the {{ScrollIntoViewOptions/block}} dictionary member of <var>options</var> to "<code>end</code>", and set the {{ScrollIntoViewOptions/inline}} dictionary member of <var>options</var> to "<code>nearest</code>".
 1. If the element does not have any associated <a>layout box</a> terminate these steps.
 1. <a lt='scroll an element into view'>Scroll the element into view</a>
     with the options <var>options</var>.
 1. Optionally perform some other action that brings the element to the user's attention.
 
+<p class=note highlight=javascript>For historical reasons, passing in <code>true</code> or
+<code>undefined</code> to {{Element/scrollIntoView()}} is equivalent to
+<code>{&nbsp;block:&nbsp;"start",&nbsp;inline:&nbsp;"nearest"&nbsp;}</code>, and passing in
+<code>false</code> is equivalent to
+<code>{&nbsp;block:&nbsp;"end",&nbsp;inline:&nbsp;"nearest"&nbsp;}</code>. Since centering is a
+common case, passing in an empty dictionary <code>{}</code> is equivalent to
+<code>{&nbsp;block:&nbsp;"center",&nbsp;inline:&nbsp;"center"&nbsp;}</code>.
+
 The <dfn method for=Element lt="scroll(options)|scroll(x, y)">scroll()</dfn> method must run these steps:
 
 1. If invoked with one argument, follow these substeps: