[Flask] Questions about Flask, Bootstrap, Web-Forms

I’m studying Flask and experimenting with it and I have some minor questions.

  1. Let’s say, I want to make a “contact us page”, should I make it with HTML5 or use Flaks’s Web Forms (package name: flask-wtf), Here are some incomplete examples:
class NameForm(FlaskForm):
  name = StringField('What is your name?', validators=[DataRequired()])
  submit = SubmitField('Submit')

@app.route('/contact', methods=['GET', 'POST'])
def contact():
  name = None
  form = NameForm()
  if form.validate_on_submit():
    name = form.name.data
    form.name.data = ''
  return render_template('contact.html', form=form, name=name)
<form method='POST'>
  {{ form.hidden_tag() }}
  {{ form.name.label }} <br>
  {{ form.name() }} <br>
  {{ form.submit() }}
    <legend class="form-group">Write your information</legend>
      <label for="fullname">Full Name:</label>
      <input type="text" class="form-control" id="fullname" placeholder="John Doe">
      <label for="email">e-mail address</label>
      <input type="email" class="form-control" id="email" placeholder="johndoe@example.com">
      <label for="message">Message</label>
      <textarea rows="5" class="form-control" id="message" placeholder="Write your message"></textarea>
      <button type="submit" class="btn btn-primary">Send</button>

When I should I use each?
( I know examples are incomplete.)

  1. Flask has also Bootstrap, should I use Flask’s Bootstrap or grab Bootstrap from a CDN and load it from HTML and where it will get downloaded by the client and my server will not have to server Bootstrap?