based on the example here: http://purecss.io/forms/
you need to set the class of the form as 'pure-form'
<form class="pure-form">
<fieldset>
your inputs
</fieldset>
</form>
alongside 'pure-form' you can add: pure-form-stacked or pure-form-aligned for different results. I recommend you to take a good look at the link above
Edit 1
Try with type='text' instead of 'textfield'
<form class="pure-form pure-form-aligned" action="{% url 'checkin' %}" method="post">
{% csrf_token %}
<fieldset>
<legend> Your position </legend>
<input type="text" name="latitude" id="latitude" placeholder="Latitude" /><br>
<input type="text" name="longitude" id="longitude" placeholder="Longitude"/><br>
<input type="text" name="accuracy" id="accuracy" placeholder="Accuracy" readonly><br>
</fieldset>
<fieldset>
<button class="pure-button " type="submit">Send </button>
</fieldset>
</form>