Allow an image to indicate its own density and correct its intrinsic …

…size.

See https://discourse.wicg.io/t/proposal-exif-image-resolution-auto-and-from-image/4326/24

This proposal allows using EXIF or other metadata to determine an image's
default resolution, which may be different from the requested resolution
in srcset/image-set, or from the default of 1dppx.

The use-case is, for example, allowing serving low-density placeholder in poor
network conditions without affecting layout.

When an image indicates its intrinsic density-corrected size, that indication
is "stronger" than the requested density (current pixel density). This allows, for example, serving a regular 1x image when a 2x image is requested by srcset, allowing for correct presentation of content that contains that information.

The change to the HTML spec doesn't define exactly how an image format
would represent this metadata, which is aligned with how image orientation
from EXIF is defined in the CSS-images spec.

The WICG proposal defines that in detail - using size+resolution from EXIF
when their values are matching.

source

@@ -27246,12 +27246,20 @@ was an English &lt;a href="/wiki/Music_hall">music hall&lt;/a> singer, ...</code
 
   <p>Each <span>image request</span> has a <dfn>current pixel density</dfn>, which must initially be undefined.</p>
 
-  <p>When an <code>img</code> element has a <span>current pixel density</span> that is not 1.0, the
-  element's image data must be treated as if its resolution, in device pixels per <span
-  data-x="'px'">CSS pixels</span>, was the <span>current pixel density</span>. The image's
-  <dfn>density-corrected intrinsic width and height</dfn> are the <span data-x="intrinsic
-  dimensions">intrinsic width and height</span> after taking into account the <span>current pixel
-  density</span>.</p>
+  <p>Each <code>img</code> element has <dfn>intrinsicly defined density-corrected dimensions</dfn>,
+  which must initially be undefined.</p>
+
+  <p>To determine the a <dfn>density-corrected intrinsic width and height</dfn> of
+  <code>img</code> element <var>img</var>:
+  <ol>
+  <li><p>Set <var>dim</var> to <var>img</var>'s <span>intrinsicly defined density-corrected
+  dimensions</span>.
+
+  <li><p>If <var>dim</var> is not undefined, return <var>dim</var>.</p></li>
+  <li><p>Set <var>dim</var> to <var>img</var>'s <span>intrinsic dimensions</span>.</p></li>
+  <li><p>Divide <var>dim</var>'s width and height by <var>img</var>'s <span>current pixel density</span>
+  <li><p>Return <var>dim</var>.
+  </ol>
 
   <p class="example">For example, if the <span>current pixel density</span> is 3.125, that means
   that there are 300 device pixels per <span data-x="'in'">CSS inch</span>, and thus if the image
@@ -27797,6 +27805,9 @@ was an English &lt;a href="/wiki/Music_hall">music hall&lt;/a> singer, ...</code
       fetched must run the following steps:</p>
 
       <ol>
+       <li><p>The user agent should attempt to determine <var>image request</var>'s image's
+       <span>intrinsicly defined density-corrected dimensions</span> from the image's metadata.
+
        <li><p>If the user agent is able to determine <var>image request</var>'s image's width and
        height, and <var>image request</var> is <span>pending request</span>, set <var>image
        request</var>'s <span data-x="img-req-state">state</span> to <span data-x="img-inc">partially
@@ -121445,6 +121456,7 @@ INSERT INTERFACES HERE
   Nikki Bee,
   Noah Mendelsohn,
   Noah Slater,
+  Noam Rosenthal,
   Noel Gordon,
   Nolan Waite,
   NoozNooz42,