title: Ensure that a user selects a value in the dropdown on submission when using Odoo html form builder
body: <p>I have a dropdown in my form which was generated by the html form builder:</p>
  
  <pre><code>&lt;select class="form-control"&gt;
      &lt;option value="" selected="1" disabled="1"&gt;Select option&lt;/option&gt;
      &lt;option value="course 1"&gt;course 1&lt;/option&gt;
      &lt;option value="course 2"&gt;course 2&lt;/option&gt;
      &lt;option value="course 3"&gt;course 3&lt;/option&gt;
  &lt;/select&gt;
  </code></pre>
  
  <p>Is there a simple way to ensure that the user selects a value in the dropdown before submitting the form? If he has not selected anything, then display an error message.</p>
  
  <p>Also, I have an input field:</p>
  
  <pre><code>&lt;div class="form-group field-x_street_name_1"&gt;
      &lt;label for="x_street_name_1" class="col-sm-2 col-form-label col-form-label-lg"&gt;Street 1 &lt;span style="color:red;"&gt;*&lt;/span&gt;&lt;/label&gt;
      &lt;div class="col-sm-10"&gt;
         &lt;input type="text" name="x_street_name_1" t-att-value="x_street_name_1" id="x_street_name_1" class="form-control form-control-lg" placeholder="e.g. John Lane" required="required" t-att-readonly="'readonly' if only_passwords else None" t-att-autofocus="'autofocus' if login and not only_passwords else None"/&gt;
      &lt;/div&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>I want to ensure the user enters only between 10 to 30 uppercase, lowercase alphabets. I would ensure that using <code>html pattern</code> simply. But it does not seem to work.</p>