Add motivating examples

Create a new motivating example section and move existing example there.
Also added two other examples related to other usecases of the API.
Add simple css markup for each example scenario.

Also formatted all lines to wrap @ 100 for consistency.

Author: majido

index.bs

@@ -33,29 +33,100 @@ Introduction {#intro}
 
 <em>This section is not normative.</em>
 
-A content author does not necessarily want <a>scroll chaining</a> to occur for all <a>scroll containers</a>. Some <a>scroll containers</a> may be part of a <a>containing block chain</a> but may serve a different logical purpose in the document and may want to prevent scrolling from continuing up the <a>scroll chain</a>. To achieve this, a content author will install event listeners without the <a>passive flag</a> set and will use <a>preventDefault</a> when there is a risk that scroll chaining will occur. This is detrimental for the following reasons:
-* The user agent may in the future introduce new input methods for scrolling that are not supported by the content author's event listeners.
-* A non passive event listener will delay scrolling because the user agent will have to wait for the result of the event listener to determine if <a>preventDefault</a> was called causing increased scroll latency.
-* When scrolling is performed near the edge of the <a>scroll boundary</a>, the <a>default action</a> may cause both scrolling to the edge of the <a>scroll container</a> and a <a>boundary default action</a>. Calling <a>preventDefault</a> will not only cancel the <a>boundary default action</a> but also the scroll to the edge of the <a>scrollport</a>.
-* The <a>default action</a> for the event may also provide additional behavior that the author does not want to cancel such as an overscroll affordance. <a>preventDefault</a> doesn't allow the content author to cancel only some of the <a>default actions</a> such as scroll chaining.
-
-Thus, it is not possible for a content author to control <a>scroll chaining</a> and overscroll in a robust, performant and forward compatible way. The 'scroll-boundary-behavior' property fixes this shortcoming.
-
-<pre class=example>
-A 'position: fixed' left navigation bar may not want to hand off scrolling to the
-document but does not want to prevent native overscroll affordances.
-</pre>
+A content author does not necessarily want <a>scroll chaining</a> to occur for all <a>scroll
+containers</a>. Some <a>scroll containers</a> may be part of a <a>containing block chain</a> but may
+serve a different logical purpose in the document and may want to prevent scrolling from continuing
+up the <a>scroll chain</a>. To achieve this, a content author will install event listeners without
+the <a>passive flag</a> set and will use <a>preventDefault</a> when there is a risk that scroll
+chaining will occur. This is detrimental for the following reasons:
+
+* The user agent may in the future introduce new input methods for scrolling that are not supported
+    by the content author's event listeners.
+* A non passive event listener will delay scrolling because the user agent will have to wait for the
+    result of the event listener to determine if <a>preventDefault</a> was called causing increased
+    scroll latency.
+* When scrolling is performed near the edge of the <a>scroll boundary</a>, the <a>default action</a>
+    may cause both scrolling to the edge of the <a>scroll container</a> and a <a>boundary default
+    action</a>. Calling <a>preventDefault</a> will not only cancel the <a>boundary default action</a>
+    but also the scroll to the edge of the <a>scrollport</a>.
+* The <a>default action</a> for the event may also provide additional behavior that the author does
+    not want to cancel such as an overscroll affordance. <a>preventDefault</a> doesn't allow the
+    content author to cancel only some of the <a>default actions</a> such as scroll chaining.
+
+Thus, it is not possible for a content author to control <a>scroll chaining</a> and overscroll in a
+robust, performant and forward compatible way. The 'scroll-boundary-behavior' property fixes this
+shortcoming.
+
+
+Motivating Examples {#motivating-examples}
+=================
+
+
+<div class=example>
+A 'position: fixed' left navigation bar does not want to hand off scrolling to the document but does
+not want to prevent native overscroll affordances. Note that A scroll gesture performed on the
+navigation bar s is almost never meant to scroll the document.
+
+  <pre class="lang-css">
+  #sidebar {
+    scroll-boundary-behavior: contain;
+  }
+  </pre>
+
+In this case, the author can use <a value for=scroll-boundary-behavior>contain</a> on the sidebar to
+prevent its scroll to chain up to document.
+</div>
+
+<div class=example>
+A page wants to implement their own pull-to-refresh effect and thus needs to disable browser
+native overscroll action.
+
+  <pre class="lang-css">
+  html {
+    /* only disable pull-to-refresh but allow swipe navigations */
+    scroll-boundary-behavior-y: contain;
+  }
+  </pre>
+
+In this case, the author can use <a value for=scroll-boundary-behavior>contain</a> on the viewport
+defining element to prevent overscroll from triggering navigation actions.
+</div>
+
+<div class=example>
+A infinite scrollers loads more content as user reaches the boundary and thus wants to disable the
+potentially confusing rubber banding effect.
+
+  <pre class="lang-css">
+  #infinite_scroller {
+    scroll-boundary-behavior-y: none;
+  }
+  </pre>
+
+In this case the the author can use <a value for=scroll-boundary-behavior>none</a> on infinite
+scroller to preventing overscroll affordance.
+</div>
 
 Scroll chaining and boundary default actions {#scroll-chaining-and-boundary-default-actions}
 ==========================
 
-<em>Operating Systems have rules for scrolling such as scroll chaining and overscroll affordances. This specification does not mandate if and how scroll chaining or overscroll affordances be implemented. This specification only allows the content author to disable them if any are implemented.</em>
+<em>Operating Systems have rules for scrolling such as scroll chaining and overscroll affordances.
+This specification does not mandate if and how scroll chaining or overscroll affordances be
+implemented. This specification only allows the content author to disable them if any are
+implemented.</em>
 
-<dfn>Scroll chaining</dfn> is when scrolling is propagated from one <a>scroll container</a> to an ancestor <a>scroll container</a> following the <a>scroll chain</a>. Typically scroll chaining is performed starting at the event target recursing up the <a>containing block chain</a>. When a <a>scroll container</a> in this chain receives a scroll event or gesture it may act on it and/or pass it up the chain. Chaining typically occurs when the <a>scrollport</a> has reached its boundary.
+<dfn>Scroll chaining</dfn> is when scrolling is propagated from one <a>scroll container</a> to an
+ancestor <a>scroll container</a> following the <a>scroll chain</a>. Typically scroll chaining is
+performed starting at the event target recursing up the <a>containing block chain</a>. When a
+<a>scroll container</a> in this chain receives a scroll event or gesture it may act on it and/or
+pass it up the chain. Chaining typically occurs when the <a>scrollport</a> has reached its boundary.
 
-A <dfn>scroll chain</dfn> is the order in which scrolling is propagated from one <a>scroll container</a> to another.
+A <dfn>scroll chain</dfn> is the order in which scrolling is propagated from one <a>scroll
+container</a> to another.
 
-<dfn>Scroll boundary</dfn> refers to when the scroll position of a <a>scroll container</a> reaches the edge of the <a>scrollport</a>. If a scroll container has no potential to scroll, because it does not <a>overflow</a> in the direction of the scroll, the element is always considered to be at the scroll boundary.
+<dfn>Scroll boundary</dfn> refers to when the scroll position of a <a>scroll container</a> reaches
+the edge of the <a>scrollport</a>. If a scroll container has no potential to scroll, because it does
+not <a>overflow</a> in the direction of the scroll, the element is always considered to be at the
+scroll boundary.
 
 <dfn>Boundary default action</dfn> refers to the user-agent-defined <a>default action</a> performed
 when scrolling against the edge of the <a>scrollport</a>. A <dfn>local boundary default action</dfn>
@@ -67,14 +138,20 @@ navigation action.
 Overview {#overview}
 ==========================
 
-This module introduces control over the behavior of a <a>scroll container</a> element when its <a>scrollport</a> reaches the boundary of its scroll box. It allows the content author to specify that a <a>scroll container</a> element must prevent scroll chaining and/or overscroll affordances.
+This module introduces control over the behavior of a <a>scroll container</a> element when its
+<a>scrollport</a> reaches the boundary of its scroll box. It allows the content author to specify
+that a <a>scroll container</a> element must prevent scroll chaining and/or overscroll affordances.
 
 Scroll Boundary Behavior Properties {#scroll-boundary-behavior-properties}
 ==========================
 
-These properties specify how a <a>scroll container</a> element must behave when scrolling. A element that is not <a>scroll container</a> must accept but ignore the values of this property. This property must be applied to all input methods supported by the user agent.
+These properties specify how a <a>scroll container</a> element must behave when scrolling. A element
+that is not <a>scroll container</a> must accept but ignore the values of this property. This
+property must be applied to all input methods supported by the user agent.
 
-Note: This property should provide guarantees that are, at least, as strong as <a>preventDefault</a> for preventing both scroll chaining and overscroll. Doing otherwise would cause content authors to use <a>preventDefault</a> instead.
+Note: This property should provide guarantees that are, at least, as strong as <a>preventDefault</a>
+for preventing both scroll chaining and overscroll. Doing otherwise would cause content authors to
+use <a>preventDefault</a> instead.
 
 <pre class=propdef>
 Name: scroll-boundary-behavior-x, scroll-boundary-behavior-y
@@ -89,7 +166,11 @@ Animatable: no
 Canonical order: <abbr title="follows order of property value definition">per grammar</abbr>
 </pre>
 
-The 'scroll-boundary-behavior-x' property specifies the behavior of the 'scroll-boundary-behavior' in the horizontal direction and the 'scroll-boundary-behavior-y' property specifies the handling of the 'scroll-boundary-behavior' in the vertical direction. When scrolling is performed along both the horizontal and vertical axes at the same time, the 'scroll-boundary-behavior' of each respective axis should be considered independently.
+The 'scroll-boundary-behavior-x' property specifies the behavior of the 'scroll-boundary-behavior'
+in the horizontal direction and the 'scroll-boundary-behavior-y' property specifies the handling of
+the 'scroll-boundary-behavior' in the vertical direction. When scrolling is performed along both the
+horizontal and vertical axes at the same time, the 'scroll-boundary-behavior' of each respective
+axis should be considered independently.
 
 <pre class=propdef>
 Name: scroll-boundary-behavior
@@ -124,6 +205,7 @@ Values have the following meanings:
     with respect to <a>scroll chaining</a>, overscroll and navigation gestures.
 </dl>
 
-Note: In the case where a user agent does not implement scroll chaining and overscroll affordances, these values will have no side effects for a compliant implementation.
+Note: In the case where a user agent does not implement scroll chaining and overscroll affordances,
+these values will have no side effects for a compliant implementation.
 
 Note: Programmatic scrolling is clamped and can not trigger any <a>boundary default actions</a>.