Clarify accName for input type=image (#497)

Author: scottaohara

index.html

@@ -6175,12 +6175,14 @@ <h4>`input type="image"` Accessible Name Computation</h4>
           <li>
             Otherwise use the associated `label` element(s) <a data-cite="accname-1.2/#dfn-accessible-name">accessible name</a>(s) - if more than one `label` is associated; concatenate by DOM order, delimited by spaces.
           </li>
-          <li>Otherwise use `alt` attribute.</li>
-          <!-- NOTE: use of valid attribute is invalid on input type=image, but it DOES contribute to the name if used -->
+          <li>Otherwise use `alt` attribute if present and its value is not the empty string.</li>
+          <!-- NOTE: use of value attribute is invalid on input type=image, but it DOES contribute to the name if used -->
           <!-- <li>Otherwise use `value` attribute.</li> -->
-          <li>Otherwise use `title` attribute.</li>
+          <li>Otherwise use `title` attribute if present and its value is not the empty string.</li>
           <li>
-            Otherwise if the previous steps do not yield a usable text string, the <a data-cite="accname-1.2/#dfn-accessible-name">accessible name</a> is a localized string of the word &quot;Submit Query&quot;.
+            Otherwise if the previous steps do not yield a usable text string, use the 
+            <a href="https://infra.spec.whatwg.org/#implementation-defined">implementation defined</a> string respective to the input type (an `input` in the `image` state
+            represents a <a data-cite="html/forms.html#concept-submit-button">submit button</a>). For instance, a localized string of the word "submit" or the words &quot;Submit Query&quot;.
           </li>
           <li>
             If none of the above yield a usable text string there is no <a data-cite="accname-1.2/#dfn-accessible-name">accessible name</a>.