We discovered an issue when a site is served via HTTP instead of HTTPS. An amp-img allows for its image to use the http protocol, so this is valid AMP:

<amp-img src="http://example.com/bison-1024x668.jpg" srcset="http://example.com/bison-1024x668.jpg 1024w, http://example.com/bison-300x196.jpg 300w, http://example.com/bison-768x501.jpg 768w, http://example.com/bison.jpg 1200w" sizes="(max-width: 767px) 89vw, (max-width: 1000px) 54vw, (max-width: 1071px) 543px, 580px" width="1024" height="668" layout="intrinsic"></amp-img>

With #1861 there was introduced noscript > img fallbacks for serving when a user with JS disabled accesses a (canonical) AMP page. This results in markup like:

<amp-img src="http://example.com/bison-1024x668.jpg" srcset="http://example.com/bison-1024x668.jpg 1024w, http://example.com/bison-300x196.jpg 300w, http://example.com/bison-768x501.jpg 768w, http://example.com/bison.jpg 1200w" sizes="(max-width: 767px) 89vw, (max-width: 1000px) 54vw, (max-width: 1071px) 543px, 580px" width="1024" height="668" layout="intrinsic">
  <noscript>
    <img src="http://example.com/bison-1024x668.jpg" srcset="http://example.com/bison-1024x668.jpg 1024w, http://example.com/bison-300x196.jpg 300w, http://example.com/bison-768x501.jpg 768w, http://example.com/bison.jpg 1200w" sizes="(max-width: 767px) 89vw, (max-width: 1000px) 54vw, (max-width: 1071px) 543px, 580px" width="1024" height="668">
  </noscript>
</amp-img>

It turns out that this actually is invalid AMP:

The src and srcset attributes are flagged as errors because—perhaps unintentionally—the noscript > img element does not allow the http protocol.

The resulting behavior is the tag-and-attribute sanitizer proceeds to remove the img. Then the AMP_Tag_And_Attribute_Sanitizer::remove_node() has a while loop to walk up the DOM tree to examine each parent, and for each empty parent, they are removed. This is a problem because noscript is an empty element is removed and amp-img is also then an empty element and is removed. The problem is that “emptiness” is not taking into account the attributes of the parent elements. So this fixes that. Nevertheless, we should also evaluate in the future if empty parents really should be removed in the first place.

An issue also needs to be opened against the AMPHTML validator to allow http protocol for noscript > img elements.

Also fixes tests which apparently break when testing on WordPress 4.9 and the latest Gutenberg by explicitly installing an old version of Gutenberg that works with 4.9.