Merge pull request #1345 from w3c/editorial/clarify-cmt

Clarify core media types

epub33/core/index.html

@@ -648,15 +648,34 @@ <h4>Introduction</h4>
 						without fallbacks (<a>Core Media Type Resources</a>) and those that cannot (<a>Foreign
 							Resources</a>).</p>
 
-					<p><a>Authors</a> are free to use both types of resources to construct their EPUB Publications, but
-						need to be aware that some Reading Systems might not render the Foreign Resources they use.</p>
-
-					<p>As EPUB Publications are designed to be fully consumable on any compliant Reading System, a
-						system of fallbacks is necessary to ensure that the use of Foreign Resources does not impact on
-						the ability of the user to consume the content. This section lists the <a
-							href="#sec-cmt-supported">set of Core Media Type Resources</a> and identifies <a
-							href="#sec-foreign-restrictions">fallback mechanisms</a> that can be used to satisfy the
-						consumability requirement.</p>
+					<p>Formats are typically only included as Core Media Type Resources when it can be shown that they
+						have broad support in web browser cores &#8212; the rendering engines on which EPUB 3 Reading
+						Systems are built. They are an agreement between Reading System developers and <a>Authors</a> to
+						ensure the predictability of rendering of EPUB Publications.</p>
+
+					<p>Inclusion as a Core Media Type Resource does not mean that all Reading Systems will support the
+						rendering of a resource, however. Only Reading Systems that are capable of rendering the type of
+						resource have to (e.g., a Reading System with a <a>Viewport</a> has to support image Core Media
+						Type Resources, but a Reading System without a Viewport does not). Refer to <a
+							href="https://www.w3.org/TR/epub-rs-33/#sec-rs-conf-general">Conformance Requirements</a>
+						[[EPUB-RS-33]] for more information about which Reading Systems rendering capabilities require
+						support for which Core Media Type Resources.</p>
+
+					<p>Foreign Resources come with no guarantee of rendering support, which is why they require a
+						fallback to a Core Media Type Resource. EPUB Publications are designed to be fully consumable on
+						any compliant Reading System, so providing a fallback is necessary to ensure that the use of
+						Foreign Resources does not impact on the ability of the user to consume the content.</p>
+
+					<p>This section lists the <a href="#sec-cmt-supported">set of Core Media Type Resources</a> and
+						identifies <a href="#sec-foreign-restrictions">fallback mechanisms</a> that can be used to
+						satisfy the consumability requirement.</p>
+
+					<div class="note">
+						<p>EPUB also exempts some [[HTML]] elements from support requirements (see <a
+								href="#sec-xhtml-fallbacks"></a>). Resources referenced from these elements are neither
+							Core Media Type Resources nor Foreign Resources &#8212; they do not require fallbacks, but
+							they also have no support requirements.</p>
+					</div>
 				</section>
 
 				<section id="sec-cmt-supported">
@@ -901,9 +920,9 @@ <h4>Foreign Resources</h4>
 
 					<ul>
 						<li>
-							<p>intrinsic fallback mechanisms provided by the format (e.g., ability to provide more than
-								one media type or to display an alternate embedded message when a media type cannot be
-								rendered);</p>
+							<p>intrinsic fallback mechanisms provided by the host format (e.g., [[HTML]] elements often
+								provide the ability to reference more than one media type or to display an alternate
+								embedded message when a media type cannot be rendered);</p>
 						</li>
 						<li>
 							<p><a href="#sec-foreign-restrictions-manifest">manifest fallbacks</a>.</p>