Clarify accName for input type=image (#497)

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>.