2.1.156 [HTML5:2014] Section 10.4.2 Images

V0357: When the image does not load, the input element of type=image does not render as a button

The specification states:

 10.4.2 Images
     ...
     User agents are expected to render img elements and input elements whose type 
     attributes are in the Image Button state, according to the first applicable rules 
     from the following list:
         ...
         If the element is an input element that does not represent an image and the user 
         agent does not expect this to change
  
             The user agent is expected to treat the element as a replaced element 
             consisting of a button whose content is the element's alternative text. The 
             intrinsic dimensions of the button are expected to be about one line in 
             height and whatever width is necessary to render the text on one line.

All document modes (All versions)

When the image does not load, the input element of type=image does not render as a button.