Add HTMLImageElement.isTypeSupported()

Give web developers a means to determine which formats are supported by the img element.

Fixes #6324.

source

@@ -28840,6 +28840,8 @@ interface <dfn interface>HTMLSourceElement</dfn> : <span>HTMLElement</span> {
 interface <dfn interface>HTMLImageElement</dfn> : <span>HTMLElement</span> {
   [<span>HTMLConstructor</span>] constructor();
 
+  static Promise&lt;boolean&gt; <span data-x="dom-img-isTypeSupported()">isTypeSupported</span>(DOMString <var>mimeType</var>);
+
   [<span>CEReactions</span>] attribute DOMString <span data-x="dom-img-alt">alt</span>;
   [<span>CEReactions</span>] attribute USVString <span data-x="dom-img-src">src</span>;
   [<span>CEReactions</span>] attribute USVString <span data-x="dom-img-srcset">srcset</span>;
@@ -29217,9 +29219,17 @@ interface <dfn interface>HTMLImageElement</dfn> : <span>HTMLElement</span> {
   </div>
 
   <dl class="domintro">
+   <dt><code data-x=""><var>supported</var> = await <span>HTMLImageElement</span>.<span subdfn data-x="dom-img-isTypeSupported">isTypeSupported</span>(<var>mimeType</var>)</code></dt>
+   <dd>
+    <p>Returns a promise resolved with true, if <var>mimeType</var> is supported for the
+    <code>img</code> element by the user agent; otherwise false.</p>
+
+    <p>The promise will be rejected with a <code>TypeError</code> if <var>mimeType</var> cannot be
+    parsed.</p>
+   </dd>
+
    <dt><code data-x=""><var>image</var>.<span subdfn data-x="dom-img-width">width</span> [ = <var>value</var> ]</code></dt>
    <dt><code data-x=""><var>image</var>.<span subdfn data-x="dom-img-height">height</span> [ = <var>value</var> ]</code></dt>
-
    <dd>
     <p>These attributes return the actual rendered dimensions of the image, or 0 if the dimensions
     are not known.</p>
@@ -29229,25 +29239,21 @@ interface <dfn interface>HTMLImageElement</dfn> : <span>HTMLElement</span> {
 
    <dt><code data-x=""><var>image</var>.<span subdfn data-x="dom-img-naturalWidth">naturalWidth</span></code></dt>
    <dt><code data-x=""><var>image</var>.<span subdfn data-x="dom-img-naturalHeight">naturalHeight</span></code></dt>
-
    <dd>
     <p>These attributes return the natural dimensions of the image, or 0 if the dimensions are not
     known.</p>
    </dd>
 
    <dt><code data-x=""><var>image</var>.<span subdfn data-x="dom-img-complete">complete</span></code></dt>
-
    <dd>
     <p>Returns true if the image has been completely downloaded or if no image is specified;
     otherwise, returns false.</p>
    </dd>
 
    <dt><code data-x=""><var>image</var>.<span subdfn data-x="dom-img-currentSrc">currentSrc</span></code></dt>
-
    <dd><p>Returns the image's <span>absolute URL</span>.</p></dd>
 
    <dt><code data-x=""><var>image</var>.<span subdfn data-x="dom-img-decode">decode</span>()</code></dt>
-
    <dd>
     <p>This method causes the user agent to <span data-x="img-decoding-process">decode</span> the
     image <span>in parallel</span>, returning a promise that fulfills when decoding is complete.</p>
@@ -29257,7 +29263,6 @@ interface <dfn interface>HTMLImageElement</dfn> : <span>HTMLElement</span> {
    </dd>
 
    <dt><code data-x=""><var>image</var> = new <span subdfn data-x="dom-image">Image</span>([ <var>width</var> [, <var>height</var> ] ])</code></dt>
-
    <dd>
     <p>Returns a new <code>img</code> element, with the <code data-x="attr-dim-width">width</code>
     and <code data-x="attr-dim-height">height</code> attributes set to the values passed in the
@@ -29267,6 +29272,44 @@ interface <dfn interface>HTMLImageElement</dfn> : <span>HTMLElement</span> {
 
   <div w-nodev>
 
+  <p>The static <dfn method for="HTMLImageElement"
+  data-x="dom-img-isTypeSupported"><code>isTypeSupported(<var>mimeType</var>)</code></dfn> method
+  steps are:</p>
+
+  <ol>
+   <li><p>Let <var>mimeTypeRecord</var> be the result of <span data-x="parse a MIME type">parsing a
+   MIME type</span> given <var>mimeType</var>.</p></li>
+
+   <li><p>If <var>mimeTypeRecord</var> is failure, then return <span>a promise rejected with</span>
+   a <code>TypeError</code>.</p></li>
+
+   <li><p>Let <var>promise</var> be a new promise.</p></li>
+
+   <li><p>Let <var>global</var> be the <span>current global object</span>.</p></li>
+
+   <li>
+    <p><span>In parallel</span>:</p>
+
+    <ol>
+     <li><p>Let <var>supported</var> be false.</p></li>
+
+     <li><p>If <var>mimeTypeRecord</var> is in the user agent's <span>supported image MIME
+     types</span> (and the user agent thus recognizes the <span data-x="MIME type
+     essence">essence</span> and all parameters (if any)), set <var>supported</var> to
+     true.</p></li>
+
+     <li><p><span>Queue a global task</span> on the <span>DOM manipulation task source</span> given
+     <var>global</var> to resolve <var>promise</var> with <var>supported</var>.</p></li>
+    </ol>
+   </li>
+
+   <li><p>Return <var>promise</var>.</p></li>
+  </ol>
+
+  <p class="note">This is modeled after <code data-x="">ImageDecoder.isTypeSupported()</code>, but
+  is its own method as user agents can support non-image formats through the <code>img</code>
+  element.</p>
+
   <p>The IDL attributes <dfn attribute for="HTMLImageElement"
   data-x="dom-img-width"><code>width</code></dfn> and <dfn attribute for="HTMLImageElement"
   data-x="dom-img-height"><code>height</code></dfn> must return the rendered width and height of the
@@ -30488,7 +30531,9 @@ was an English &lt;a href="/wiki/Music_hall">music hall&lt;/a> singer, ...</code
   of a multipage resource (e.g. a PDF file). User agents must not allow the resource to act in an
   interactive fashion, but should honour any animation in the resource.</p>
 
-  <p>This specification does not specify which image types are to be supported.</p>
+  <p>A user agent has an associated <dfn>supported image MIME types</dfn> (a <span>set</span> of
+  <span data-x="MIME type">MIME types</span>). A user agent's <span>supported image MIME
+  types</span> is <span>implementation-defined</span>.</p>
 
 
   <h6>When to obtain images</h6>
@@ -141942,6 +141987,7 @@ INSERT INTERFACES HERE
   Matthew Gregan,
   Matthew Mastracci,
   Matthew Noorenberghe,
+  Matthew Petroff,
   Matthew Raymond,
   Matthew Thomas,
   Matthew Tylee Atkinson, <!-- matatk on GitHub -->