Make <img src> optional in presence of srcset

Fixes #9181.

Author: annevk

source

@@ -29384,13 +29384,16 @@ interface <dfn interface>HTMLImageElement</dfn> : <span>HTMLElement</span> {
   any previous sibling <code>source</code> elements' <code data-x="attr-source-srcset">srcset</code>
   attributes if the parent is a <code>picture</code> element, is the embedded content; the value of
   the <dfn element-attr for="img" data-x="attr-img-alt"><code>alt</code></dfn> attribute provides
-  equivalent content for those who cannot process images or who have image loading disabled (i.e. it
-  is the <code>img</code> element's <span>fallback content</span>).</p>
+  equivalent content for those who cannot process images or who have image loading disabled (i.e.,
+  it is the <code>img</code> element's <span>fallback content</span>).</p>
 
   <p>The requirements on the <code data-x="attr-img-alt">alt</code> attribute's value are described
   <a href="#alt">in a separate section</a>.</p>
 
-  <p>The <code data-x="attr-img-src">src</code> attribute must be present, and must contain a
+  <p>At least one of the <code data-x="attr-img-src">src</code> and <code
+  data-x="attr-img-srcset">srcset</code> attributes must be present.</p>
+
+  <p>If the <code data-x="attr-img-src">src</code> attribute is present, it must contain a
   <span>valid non-empty URL potentially surrounded by spaces</span> referencing a non-interactive,
   optionally animated, image resource that is neither paged nor scripted.</p>
 
@@ -29400,10 +29403,10 @@ interface <dfn interface>HTMLImageElement</dfn> : <span>HTMLElement</span> {
   <span>document element</span> that use declarative SMIL animation), and so forth. However, these
   definitions preclude SVG files with script, multipage PDF files, interactive MNG files, HTML
   documents, plain text documents, and the like. <ref>PNG</ref> <ref>GIF</ref> <ref>JPEG</ref>
-  <ref>PDF</ref> <ref>XML</ref> <ref>APNG</ref> <ref>SVG</ref> <ref>MNG</ref> </p>
+  <ref>PDF</ref> <ref>XML</ref> <ref>APNG</ref> <ref>SVG</ref> <ref>MNG</ref></p>
 
-  <p>The <code data-x="attr-img-srcset">srcset</code> attribute may also be present, and is a
-  <span>srcset attribute</span>.</p>
+  <p>The <code data-x="attr-img-srcset">srcset</code> attribute is a <span>srcset
+  attribute</span>.</p>
 
   <p>The <code data-x="attr-img-srcset">srcset</code> attribute and the <code
   data-x="attr-img-src">src</code> attribute (if <span data-x="width descriptor">width