[cssom-view] Allow UAs to lie about the screen information

Fixes
https://www.w3.org/Bugs/Public/show_bug.cgi?id=29577
https://www.w3.org/Bugs/Public/show_bug.cgi?id=29578
https://www.w3.org/Bugs/Public/show_bug.cgi?id=29579
https://www.w3.org/Bugs/Public/show_bug.cgi?id=29580

Author: zcorpan

cssom-view/Overview.bs

@@ -88,6 +88,7 @@ url: https://www.w3.org/TR/DOM-Level-3-Events/#interface-MouseEvent; type: inter
 spec:css-display-3; type:value; for:display; text:table
 spec:css-position-3; type:property; text:position
 spec:dom; type:interface; text:Document
+spec:html; type:dfn; text:case-sensitive
 </pre>
 
 <script src=https://resources.whatwg.org/file-issue.js defer data-file-issue-url="https://github.com/w3c/csswg-drafts/issues/new?title=%5Bcssom-view%5D%20"></script>
@@ -353,6 +354,26 @@ There are two kinds of zoom, <dfn>page zoom</dfn> which affects the size of the
 a magnifying glass and does not affect the initial viewport<!-- XXX ref --> or actual viewport<!-- XXX ref -->. [[!CSS-DEVICE-ADAPT]]
 
 
+Web-exposed screen information {#web-exposed-screen-information}
+----------------------------------------------------------------
+
+User agents may choose to hide information about the screen of the output device, in order to
+protect the user's privacy. In order to do so in a consistent manner across APIs, this specification
+defines the following terms, each having a width and a height, the origin being the top left
+corner, and the x- and y-coordinates increase rightwards and downwards, respectively.
+
+The <dfn export>Web-exposed screen area</dfn> is one of the following:
+
+* The area of the output device, in CSS pixels.
+* The area of the <a>viewport</a>, in CSS pixels.
+
+The <dfn export>Web-exposed available screen area</dfn> is one of the following:
+
+* The available area of the rendering surface of the output device, in CSS pixels.
+* The area of the output device, in CSS pixels.
+* The area of the <a>viewport</a>, in CSS pixels.
+
+
 Common Infrastructure {#common-infrastructure}
 ==============================================
 
@@ -609,12 +630,12 @@ user agent must run these steps:
 1. Act as if the {{Window/scroll()}} method was invoked with <var>options</var> as the only argument.
 
 The <dfn attribute for=Window>screenX</dfn> attribute must return the x-coordinate,
-relative to the origin of the screen of the output device, of the left of
+relative to the origin of the <a>Web-exposed screen area</a>, of the left of
 the client window as number of pixels, or zero if there is no such
 thing. <!--fingerprint-->
 
 The <dfn attribute for=Window>screenY</dfn> attribute must return the y-coordinate,
-relative to the origin of the screen of the output device, of the top of
+relative to the origin of the screen of the <a>Web-exposed screen area</a>, of the top of
 the client window as number of pixels, or zero if there is no such
 thing. <!--fingerprint-->
 
@@ -864,15 +885,17 @@ interface Screen {
 };
 </pre>
 
-The <dfn attribute for=Screen>availWidth</dfn> attribute must return the available width of the rendering surface of the output device,
-in CSS pixels. <!--fingerprint-->
+The <dfn attribute for=Screen>availWidth</dfn> attribute must return the width of the <a>Web-exposed
+available screen area</a>. <!--fingerprint-->
 
-The <dfn attribute for=Screen>availHeight</dfn> attribute must return the available height of the rendering surface of the output
-device, in CSS pixels. <!--fingerprint-->
+The <dfn attribute for=Screen>availHeight</dfn> attribute must return the height of the
+<a>Web-exposed available screen area</a>. <!--fingerprint-->
 
-The <dfn attribute for=Screen>width</dfn> attribute must return the width of the output device, in CSS pixels. <!--fingerprint-->
+The <dfn attribute for=Screen>width</dfn> attribute must return the width of the <a>Web-exposed
+screen area</a>. <!--fingerprint-->
 
-The <dfn attribute for=Screen>height</dfn> attribute must return the height of the output device, in CSS pixels. <!--fingerprint-->
+The <dfn attribute for=Screen>height</dfn> attribute must return the height of the <a>Web-exposed
+screen area</a>. <!--fingerprint-->
 
 The <dfn attribute for=Screen>colorDepth</dfn> attribute must return 24.
 
@@ -1481,11 +1504,11 @@ partial dictionary MouseEventInit {
 
 The <dfn attribute for=MouseEvent>screenX</dfn> attribute must return the x-coordinate of
 the position where the event occurred relative to the origin of the
-screen.
+<a>Web-exposed screen area</a>.
 
 The <dfn attribute for=MouseEvent>screenY</dfn> attribute must return the y-coordinate of
 the position where the event occurred relative to the origin of the
-screen.
+<a>Web-exposed screen area</a>.
 
 The <dfn attribute for=MouseEvent>pageX</dfn> attribute must follow these steps: