[css-viewport] [cssom-view-1] Add a definition of the zoom CSS property

css-viewport/Overview.bs

@@ -25,6 +25,10 @@ spec: virtual-keyboard; urlPrefix: https://w3c.github.io/virtual-keyboard
 	type: attribute; text: overlaysContent; for: VirtualKeyboard; url: dom-virtualkeyboard-overlayscontent
 </pre>
 
+<pre class="anchors">
+spec: fenced-frames; urlPrefix: https://wicg.github.io/fenced-frame/
+	type: interface; text: FencedFrames
+</pre>
 
 <h2 id="intro">
 Introduction</h2>
@@ -356,6 +360,157 @@ only the value previously set to it.
 }
 </pre>
 
+<h2 id='zoom-property'>
+	The 'zoom' property
+</h2>
+
+An element becomes zoomed when the 'zoom' property has a positive computed value different than 1 (or when a flat tree ancestor has zoom).
+'zoom' affects computed property values and inherited property values.
+The computed value of 'zoom' is applied as a scalar to the used value of <<length>> CSS properties for the element and its descendants
+(including inherited properties such as 'line-height' or 'font-size'),
+plus intrinsic sizing of all replaced elements,
+background images,
+and nested frames (except for fenced frames [[!FENCED-FRAME]]),
+resulting in a magnification or minification effect.
+
+Nested values of 'zoom' multiply, resulting in additional scaling of <<length>> values.
+This means that the used value for zoom is always its [=effective zoom=].
+
+The 'zoom' property has no effect on <<length>> property values with computed values that are 'auto' or <<percentage>>.
+
+Note: Unlike 'transform',
+scaling the 'zoom' property affects layout.
+
+Note: The computed value of 'font-size' is never <<percentage>>;
+thus 'zoom' always applies.
+
+Note: 'zoom' does not affect or prevent 'transform' scaling.
+
+<pre class="propdef">
+	Name: zoom
+	Value: <<number>> || <<percentage>>
+	Initial: 1
+	Applies to: all <<length>> property values of all elements
+	Inherited: no
+	Percentages: Converted to <<number>>
+	Media: visual
+	Computed value: as specified, but with <<percentage>> converted to the equivalent <<number>>
+	Animation type: not animatable
+</pre>
+
+The values of this property have the following meanings:
+
+<dl dfn-for="zoom" dfn-type=value>
+	<dt><dfn><<number>></dfn>
+	<dd>
+		Positive floating point number indicating a zoom factor.
+		Numbers smaller than 1.0 indicate a "zoom out" or minification effect,
+		while numbers greater than 1.0 indicate a "zoom in" or magnification effect.
+		A 0 value is treated as if it was 1.
+
+		Note: The treatment of 0 is a web compatibility quirk.
+
+	<dt><dfn><<percentage>></dfn>
+	<dd>
+		Positive floating point number,
+		followed by a percentage character ("%") which indicates a zoom factor multiplied by 100.
+		A 0 percentage is treated as if it was 100%.
+
+		Note: The treatment of 0 is a web compatibility quirk.
+
+</dl>
+
+Negative values for 'zoom' are illegal.
+
+<div class="example">
+	Example of the 'zoom' property applied during hover for magnification effect.
+
+	<pre>
+		&lt;div class="messageBox"&gt;
+			&lt;div class="label"&gt;Text of the label&lt;/div&gt;
+		&lt;/div&gt;
+
+		&lt;style&gt;
+		.messageBox {
+		    width: 10em;
+		    padding: 2em;
+		    border: medium solid lightblue;
+		}
+
+		.messageBox:hover {
+		    zoom: 150%;
+		}
+
+		.label {
+		    background: lightgrey;
+		    padding: 1em;
+		    text-align: center;
+		}
+		&lt;/style&gt;
+	</pre>
+
+	Here is an llustration of the before and after hover state of the message box element:
+	<img src="css_zoom_hover_example.png" alt="Two images,
+	showing the zooming effect before and after zoom has applied. The second is 1.5 larger.">
+</div>
+
+<div class="example">
+	Example of nested zoom.
+	In this example, "Inner text" is 4x as large as "Outer text",
+	and "Middle text" is 2x as large as "Outer text".
+
+	<pre>
+		&lt;div style="zoom: 2"&gt;
+		  Middle text
+			&lt;div style="zoom: 2"&gt;
+			   Inner text
+			&lt;div&gt;
+		&lt;div&gt;
+		Outer text
+	</pre>	 
+</div>
+
+<div class="example">
+	Example of replaced elements. In this example,
+	the image and iframe will be twice as large as their default sizing.
+
+	<pre>
+	&lt;div style="zoom: 2"&gt;
+		&lt;img src="..."&gt;
+		&lt;iframe src="..."&gt;&lt;/iframe&gt;
+	&lt;div&gt;
+	</pre>	 
+</div>
+
+The <dfn>effective zoom</dfn> of an element is the product of its computed
+value of 'zoom' and all ancestors' computed values of 'zoom'.
+
+The <dfn export>scaled</dfn> value of a CSS length is the used value of that length;
+in particular it includes zoom.
+
+The <dfn export>unscaled</dfn> value of a CSS length relative to an element is the [=scaled=] value divided by the element's [=effective zoom=].
+
+<div class="note">
+	The [=effective zoom=] of an element in a nested frame may be a value other than 1 even if 'zoom' is never set on an element in that frame. This can be observed by authors via APIs such as {{Window/devicePixelRatio}} and {{Element/getBoundingClientRect}}.
+</div>
+
+<h3 id='zoom-om'>
+	DOM and CSSOM interaction
+</h3>
+
+	Computed style APIs (i.e., all values returned by {{getComputedStyle()}}) that are non-auto and non-percentage lengths must be [=unscaled=].
+
+	The {{Element/getBoundingClientRect}},
+	{{Element/getClientRects}},
+	and {{IntersectionObserver}} APIs must return rects with [=scaled=]
+	lengths.
+
+	All other APIs related to element geometries must return [=unscaled=] lengths.
+	This is explained in detail in [[cssom-view#extensions-to-the-htmlelement-interface]].
+
+	The {{Window/devicePixelRatio}} of a frame is multiplied by the [=effective zoom=] inherited by its parent frame.
+</h3>
+
 <h2 class="no-num" id="changes">Appendix A. Changes</h2>
 
 This appendix is <em>informative</em>.

cssom-view-1/Overview.bs

@@ -1141,7 +1141,7 @@ aborting on the first step that returns a value:
     1. Return the {{DOMRect}} object in <var>list</var> at index 0.
 1. If <a>caret node</a> is a text entry widget that is a replaced element,
     and that is in the document,
-    return a {{DOMRect}} object for the caret in the widget
+    return a [=scaled=] {{DOMRect}} object for the caret in the widget
     as represented by the <a>caret offset</a> value.
     The <a>transforms</a> that apply to the element and its ancestors are applied.
 1. Return null.
@@ -1190,7 +1190,7 @@ partial interface Element {
 The <dfn method for=Element>getClientRects()</dfn> method, when invoked, must return the result of the following algorithm:
 
 1. If the element on which it was invoked does not have an associated [=CSS/box=] return an empty {{DOMRectList}} object and stop this algorithm.
-1. If the element has an associated <a>SVG layout box</a> return a {{DOMRectList}} object containing a single {{DOMRect}} object that describes the bounding box of the element as defined by the SVG specification, applying the <a>transforms</a> that apply to the element and its ancestors.
+1. If the element has an associated <a>SVG layout box</a> return a [=scaled=] {{DOMRectList}} object containing a single {{DOMRect}} object that describes the bounding box of the element as defined by the SVG specification, applying the <a>transforms</a> that apply to the element and its ancestors.
 1. Return a {{DOMRectList}} object containing {{DOMRect}} objects in content order, one for each <a spec=css-break>box fragment</a>, describing its border area (including those with a height or width of zero) with the following constraints:
 
     * Apply the <a>transforms</a> that apply to the element and its ancestors.
@@ -1401,24 +1401,24 @@ The <dfn attribute for=Element>scrollHeight</dfn> attribute must return the resu
 The <dfn attribute for=Element>clientTop</dfn> attribute must run these steps:
 
 1. If the element has no associated [=CSS/box=] or if the [=CSS/box=] is inline, return zero.
-1. Return the computed value of the 'border-top-width' property plus the height of any scrollbar rendered between the top <a>padding edge</a> and the top <a>border edge</a>, ignoring any <a>transforms</a> that apply to the element and its ancestors.
+1. Return the [=unscaled=] computed value of the 'border-top-width' property plus the height of any scrollbar rendered between the top <a>padding edge</a> and the top <a>border edge</a>, ignoring any <a>transforms</a> that apply to the element and its ancestors.
 
 The <dfn attribute for=Element>clientLeft</dfn> attribute must run these steps:
 
 1. If the element has no associated [=CSS/box=] or if the [=CSS/box=] is inline, return zero.
-1. Return the computed value of the 'border-left-width' property plus the width of any scrollbar rendered between the left <a>padding edge</a> and the left <a>border edge</a>, ignoring any <a>transforms</a> that apply to the element and its ancestors.
+1. Return the [=unscaled=] computed value of the 'border-left-width' property plus the width of any scrollbar rendered between the left <a>padding edge</a> and the left <a>border edge</a>, ignoring any <a>transforms</a> that apply to the element and its ancestors.
 
 The <dfn attribute for=Element>clientWidth</dfn> attribute must run these steps:
 
 1. If the element has no associated [=CSS/box=] or if the [=CSS/box=] is inline, return zero.
 1. If the element is the [=root element=] and the element's <a>node document</a> is not in <a>quirks mode</a>, or if the element is <a>the <code>body</code> element</a> and the element's <a>node document</a> <em>is</em> in <a>quirks mode</a>, return the <a>viewport</a> width excluding the size of a rendered scroll bar (if any).
-1. Return the width of the <a>padding edge</a> excluding the width of any rendered scrollbar between the <a>padding edge</a> and the <a>border edge</a>, ignoring any <a>transforms</a> that apply to the element and its ancestors.
+1. Return the [=unscaled=] width of the <a>padding edge</a> excluding the width of any rendered scrollbar between the <a>padding edge</a> and the <a>border edge</a>, ignoring any <a>transforms</a> or that apply to the element and its ancestors.
 
 The <dfn attribute for=Element>clientHeight</dfn> attribute must run these steps:
 
 1. If the element has no associated [=CSS/box=] or if the [=CSS/box=] is inline, return zero.
 1. If the element is the [=root element=] and the element's <a>node document</a> is not in <a>quirks mode</a>, or if the element is <a>the <code>body</code> element</a> and the element's <a>node document</a> <em>is</em> in <a>quirks mode</a>, return the <a>viewport</a> height excluding the size of a rendered scroll bar (if any).
-1. Return the height of the <a>padding edge</a> excluding the height of any rendered scrollbar between the <a>padding edge</a> and the <a>border edge</a>, ignoring any <a>transforms</a> that apply to the element and its ancestors.
+1. Return the [=unscaled=] height of the <a>padding edge</a> excluding the height of any rendered scrollbar between the <a>padding edge</a> and the <a>border edge</a>, ignoring any <a>transforms</a> that apply to the element and its ancestors.
 
 
 <h3 id=element-scrolling-members>{{Element}} Scrolling Members</h3>
@@ -1536,6 +1536,7 @@ The <dfn attribute for=HTMLElement>offsetParent</dfn> attribute must return the
     1. If <var>ancestor</var> is <a>closed-shadow-hidden</a> from the element and its computed value of the 'position' property is ''position/fixed'', terminate this algorithm and return null.
     1. If <var>ancestor</var> is not <a>closed-shadow-hidden</a> from the element and satisfies at least one of the following, terminate this algorithm and return <var>ancestor</var>.
         * The element is a containing block of absolutely-positioned descendants (regardless of whether there are any absolutely-positioned descendants).
+        * The element has a non-default used value of 'zoom'.
         * It is <a>the <code>body</code> element</a>.
         * The computed value of the 'position' property of the element is ''static'' and the ancestor is one of the following <a>HTML elements</a>: <code>td</code>, <code>th</code>, or <code>table</code>.
     1. If there is no more parent of <var>ancestor</var> in the <a>flat tree</a>, terminate this algorithm and return null.
@@ -1544,8 +1545,8 @@ The <dfn attribute for=HTMLElement>offsetParent</dfn> attribute must return the
 The <dfn attribute for=HTMLElement>offsetTop</dfn> attribute must return the result of running these steps:
 
 1. If the element is <a>the <code>body</code> element</a> or does not have any associated [=CSS/box=] return zero and terminate this algorithm.
-1. If the {{HTMLElement/offsetParent}} of the element is null return the y-coordinate of the top <a>border edge</a> of the first [=CSS/box=] associated with the element, relative to the <a>initial containing block</a> origin, ignoring any <a>transforms</a> that apply to the element and its ancestors, and terminate this algorithm.
-1. Return the result of subtracting the y-coordinate of the top <a>padding edge</a>
+1. If the {{HTMLElement/offsetParent}} of the element is null return the [=unscaled=] y-coordinate of the top <a>border edge</a> of the first [=CSS/box=] associated with the element, relative to the <a>initial containing block</a> origin, ignoring any <a>transforms</a>that apply to the element and its ancestors and terminate this algorithm.
+1. Return the [=unscaled=] result of subtracting the y-coordinate of the top <a>padding edge</a>
     of the first [=CSS/box=] associated with the {{HTMLElement/offsetParent}} of the element
     from the y-coordinate of the top <a>border edge</a>
     of the first [=CSS/box=] associated with the element,
@@ -1557,13 +1558,13 @@ The <dfn attribute for=HTMLElement>offsetTop</dfn> attribute must return the res
 The <dfn attribute for=HTMLElement>offsetLeft</dfn> attribute must return the result of running these steps:
 
 1. If the element is <a>the <code>body</code> element</a> or does not have any associated [=CSS/box=] return zero and terminate this algorithm.
-1. If the {{HTMLElement/offsetParent}} of the element is null return the x-coordinate of the left <a>border edge</a> of the first [=CSS/box=] associated with the element, relative to the <a>initial containing block</a> origin, ignoring any <a>transforms</a> that apply to the element and its ancestors, and terminate this algorithm.
-1. Return the result of subtracting the x-coordinate of the left <a>padding edge</a> of the first [=CSS/box=] associated with the {{HTMLElement/offsetParent}} of the element from the x-coordinate of the left <a>border edge</a> of the first [=CSS/box=] associated with the element, relative to the <a>initial containing block</a> origin, ignoring any <a>transforms</a> that apply to the element and its ancestors.
+1. If the {{HTMLElement/offsetParent}} of the element is null return the [=unscaled=] x-coordinate of the left <a>border edge</a> of the first [=CSS/box=] associated with the element, relative to the <a>initial containing block</a> origin, ignoring any <a>transforms</a> that apply to the element and its ancestors, and terminate this algorithm.
+1. Return the [=unscaled=] result of subtracting the x-coordinate of the left <a>padding edge</a> of the first [=CSS/box=] associated with the {{HTMLElement/offsetParent}} of the element from the x-coordinate of the left <a>border edge</a> of the first [=CSS/box=] associated with the element, relative to the <a>initial containing block</a> origin, ignoring any <a>transforms</a> that apply to the element and its ancestors.
 
 The <dfn attribute for=HTMLElement>offsetWidth</dfn> attribute must return the result of running these steps:
 
 1. If the element does not have any associated [=CSS/box=] return zero and terminate this algorithm.
-1. Return the width of the axis-aligned bounding box
+1. Return the [=unscaled=] width of the axis-aligned bounding box
     of the [=border boxes=]
     of all fragments generated by the element's [=principal box=],
     ignoring any <a>transforms</a> that apply to the element and its ancestors.
@@ -1576,7 +1577,7 @@ The <dfn attribute for=HTMLElement>offsetWidth</dfn> attribute must return the r
 The <dfn attribute for=HTMLElement>offsetHeight</dfn> attribute must return the result of running these steps:
 
 1. If the element does not have any associated [=CSS/box=] return zero and terminate this algorithm.
-1. Return the height of the axis-aligned bounding box
+1. Return the [=unscaled=] height of the axis-aligned bounding box
     of the [=border boxes=]
     of all fragments generated by the element's [=principal box=],
     ignoring any <a>transforms</a> that apply to the element and its ancestors.
@@ -1596,11 +1597,11 @@ partial interface HTMLImageElement {
 };
 </pre>
 
-The <dfn attribute for=HTMLImageElement>x</dfn> attribute, on getting, must return the x-coordinate of the left <a>border edge</a> of the
+The <dfn attribute for=HTMLImageElement>x</dfn> attribute, on getting, must return the [=scaled=] x-coordinate of the left <a>border edge</a> of the
 first [=CSS/box=] associated with the element, relative to the <a>initial containing block</a> origin, ignoring any
 <a>transforms</a> that apply to the element and its ancestors, or zero if there is no [=CSS/box=].
 
-The <dfn attribute for=HTMLImageElement>y</dfn> attribute, on getting, must return the y-coordinate of the top <a>border edge</a> of the
+The <dfn attribute for=HTMLImageElement>y</dfn> attribute, on getting, must return the [=scaled=] y-coordinate of the top <a>border edge</a> of the
 first [=CSS/box=] associated with the element, relative to the <a>initial containing block</a> origin, ignoring any
 <a>transforms</a> that apply to the element and its ancestors, or zero if there is no [=CSS/box=].
 
@@ -1620,7 +1621,7 @@ containing a list of {{DOMRect}} objects in content order that matches the follo
 
 * For each element selected by the range, whose parent is not selected by the range, include the border areas returned by invoking {{Element/getClientRects()}} on the element.
 * For each {{Text}} node selected or partially selected by the range (including when the
-    boundary-points are identical), include a {{DOMRect}} object (for the part that is selected, not
+    boundary-points are identical), include [=scaled=] {{DOMRect}} object (for the part that is selected, not
     the whole line box). The bounds of these {{DOMRect}} objects are computed using font metrics;
     thus, for horizontal writing, the vertical dimension of each box is determined by the font
     ascent and descent, and the horizontal dimension by the text advance width. If the range covers