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<link rel="stylesheet" type='text/css" href="{{ url_for('static', filename='main.css') }}">
Make sure that the css is in the 'static' file you should have created
inside the root directory of the project.
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<link rel="stylesheet" href="{{ url_for('static', filename='css/main.css') }}">
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<link rel= "stylesheet" type= "text/css" href= "{{ url_for('static',filename='styles/mainpage.css') }}">
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{% block styles %}
{{super()}}
<link rel="stylesheet"
href="{{url_for('.static', filename='mystyle.css')}}">
{% endblock %}