title: html required attribute does not work
body: <p>I use Angular cli for a simple form to submit, however, the required attributes in the input elements do not work, I can still submit the form without inputting anything, even though they are wrapped in the form element. I googled the solutions, but somehow I can't still figure it out. Thank you in advance.</p>
  
  <p>Here is the html code:</p>
  
  <pre><code>&lt;form (submit)="onAddSubmit()"&gt;
    &lt;div class="form-group"&gt;
      &lt;label&gt;Title&lt;/label&gt;
      &lt;input class="form-control" type="text" [(ngModel)]="title" name="title" required&gt;
    &lt;/div&gt;
    &lt;div class="form-group"&gt;
      &lt;label&gt;City&lt;/label&gt;
      &lt;input class="form-control" type="text" [(ngModel)]="city" name="city" required&gt;
    &lt;/div&gt;
    &lt;div class="form-group"&gt;
      &lt;label&gt;Owner&lt;/label&gt;
      &lt;input class="form-control" type="text" [(ngModel)]="owner" name="owner" required&gt;
    &lt;/div&gt;
    &lt;div class="form-group"&gt;
      &lt;label&gt;Bedrooms&lt;/label&gt;
      &lt;input class="form-control" type="number" [(ngModel)]="bedrooms" name="bedrooms" required&gt;
    &lt;/div&gt;
    &lt;div class="form-group"&gt;
      &lt;label&gt;Type&lt;/label&gt;
      &lt;select class="form-control" type="text" [(ngModel)]="type" name="type" required&gt;
        &lt;option value="Estate"&gt;Estate&lt;/option&gt;
        &lt;option value="Condo"&gt;Condo&lt;/option&gt;
        &lt;option value="Apartment"&gt;Apartment&lt;/option&gt;
      &lt;/select&gt;
    &lt;/div&gt;
    &lt;div class="form-group"&gt;
      &lt;input id="image" name="image" [(ngModel)]="image" type="file" &gt;
    &lt;/div&gt;
    &lt;div class="form-group"&gt;
      &lt;label&gt;Price&lt;/label&gt;
      &lt;input class="form-control" type="text" [(ngModel)]="price" name="price" required&gt;
    &lt;/div&gt;
    &lt;input type="submit" value="Submit" class="btn btn-success"&gt;
  &lt;/form&gt;
  </code></pre>