Add section on native browser form validation to 3.3.1 understanding

understanding/20/error-identification.html

@@ -78,6 +78,26 @@ <h2>Intent of Error Identification</h2>
 
       <p>See also <a href="error-suggestion">3.3.3: Error Suggestion</a>.</p>
 
+      <h3>User agent native HTML form validation</h3>
+
+      <p>When using native HTML <a href="https://html.spec.whatwg.org/multipage/forms.html#client-side-form-validation">client-side form validation</a>,
+         user agents will automatically prevent the submission of incomplete or invalid forms, and display generic error messages to the user.
+         The user agent will generally set focus back to the first form field that is in error, and as a result 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 announce the error message
+         and the programmatic name of the focused field. 
+         While this 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 not be permanent, or fail to scroll with the page.</li>
+         <li>Depending on the user agent, even if a user has zoomed-in (magnified) the content, the error messages will not appear magnified,
+            as the text in the validation message will be displayed at the same size as the user agent interface; the message may be too small for users to read.</li>
+         <li>The default HTML validation error messages are generally quite generic, and they may not provide sufficiently helpful or specific suggestions to the user
+            that would conform to <a href="error-suggestion">3.3.3 Error Suggestion</a>.
+         </li>
+         <li>If several errors are present, only the first error message 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>
+
    </section>
    <section id="benefits">
       <h2>Benefits of Error Identification</h2>