Why is static_url not working?

Question

Django

In settings.py I have either
STATIC_URL = '/static/'

STATIC_HOST = os.environ.get('DJANGO_STATIC_HOST', '')
STATIC_URL = STATIC_HOST + '/static/'

And in html

{% load static %}
<link href="{% static "d.css" %}" rel="stylesheet" type="text/css" media="screen" />

But I am getting an error

127.0.0.1/:12 GET http://127.0.0.1:8000/static/d.css net::ERR_ABORTED 404 (Not Found)

What have I done wrong?