Add section on native browser form validation to 3.3.1 understanding

understanding/20/error-identification.html

@@ -47,7 +47,24 @@ <h2>Intent of Error Identification</h2>
          <li>the user enters alphabetic characters or parentheses into their phone number field that only accepts numbers;</li>
          <li>the user enters a bid that is below the previous bid or the minimum bid increment.</li>
       </ul>
-
+<h3>Note on using native HTML form validation</h3>
+<p>With native HTML form validation, the use of the <code>required</code> attribute on a field has the consequence that 
+     when trying to submit the form with this field empty, the user agent will automatically display a generic error message that a value 
+  in the field is required, and set the focus to the first field missing required input.
+  The user agent will generally also scroll the page so that the field in error and the generated error message will be visible in the viewport.</p> 
+  <p>In most common user agent and screen reader combinations, the screen reader will read the generic error message 
+    and the programmatic name of the required field. 
+    While this in principle meets the requirements of this Success Criterion, 
+    it should be noted that there are several disadvantages related to this approach:</p>
+     <ul>
+         <li>Depending on the user agent, the message may be non-permanent, or fail to scroll with the page</li>
+         <li>Depending on the user agent, zoomed-in (magnified) content may display the error message unmagnified; 
+  the message may may be too small for users to read</li>
+         <li>HTML validation messages for fields with <code>type="text"</code> will be unspecific, i.e. they will not provide helpful suggestions where these may be needed</li>
+         <li>If several errors are present, only the first error is exposed. 
+  Once the user has provided an input that conforms to the type of field and resubmits the form, 
+  the next error (if present) will be exposed. This means that repeated resubmissions and corrections may be required.</li>
+     </ul>
       <div class="note">
          <p>If a user enters a value that is too high or too low, and the coding on the page automatically
             changes that value to fall within the allowed range, the user's error would still