Make <img>.complete more reliable

These changes are in line with all three browsers. Chrome still diverges significantly from the standard & tests though on other aspects.

Tests: web-platform-tests/wpt#19298.

Fixes #1055, fixes #4475. #4476 remains outstanding.

source

@@ -26519,7 +26519,6 @@ interface <dfn>HTMLImageElement</dfn> : <span>HTMLElement</span> {
   any of the following conditions is true:</p>
 
   <ul class="brief">
-
    <!--TODO: check picture parent-->
 
    <li>Both the <code data-x="attr-img-src">src</code> attribute and the <code
@@ -26530,23 +26529,17 @@ interface <dfn>HTMLImageElement</dfn> : <span>HTMLElement</span> {
    hack for src="", not srcset=""; present but empty or bogus srcset="" still means complete=false
    if it's not in the img-error state -->
 
-   <li>The final <span data-x="concept-task">task</span> that is <span data-x="queue a
-   task">queued</span> by the <span>networking task source</span> once the resource has been fetched
-   has been <span data-x="queue a task">queued</span>.
-
    <li>The <code>img</code> element's <span>current request</span>'s <span
-   data-x="img-req-state">state</span> is <span data-x="img-all">completely available</span>.
+   data-x="img-req-state">state</span> is <span data-x="img-all">completely available</span> and its
+   <span>pending request</span> is null.
 
    <li>The <code>img</code> element's <span>current request</span>'s <span
-   data-x="img-req-state">state</span> is <span data-x="img-error">broken</span>.
-
+   data-x="img-req-state">state</span> is <span data-x="img-error">broken</span> and its
+   <span>pending request</span> is null.
   </ul>
 
   <p>Otherwise, the attribute must return false.</p>
 
-  <p class="note">The value of <code data-x="dom-img-complete">complete</code> can thus change while
-  a <span data-x="concept-script">script</span> is executing.</p>
-
   <p>The <dfn data-x="dom-img-currentSrc"><code>currentSrc</code></dfn> IDL attribute
   must return the <code>img</code> element's <span>current request</span>'s <span data-x="img-req-url">current URL</span>.</p>
 
@@ -26573,7 +26566,8 @@ img.decode();</code></pre>
     <ol>
      <!-- TODO: much of this could be phrased entirely in terms of the current request if
      https://github.com/whatwg/html/issues/1055 gets fixed. See
-     https://github.com/whatwg/html/pull/2332#issuecomment-278613713 -->
+     https://github.com/whatwg/html/pull/2332#issuecomment-278613713
+     XXX Domenic what to do about this comment? -->
      <li>
       <p>If any of the following conditions are true about this <code>img</code> element:</p>