moviewyrm/bookwyrm/templates/snippets/tag.html

<div class="control">
    <form name="tag" action="/{% if tag.tag.identifier in user_tags %}untag{% else %}tag{% endif %}/" method="post">
        {% csrf_token %}
        <input type="hidden" name="book" value="{{ book.id }}">
        <input type="hidden" name="name" value="{{ tag.tag.name }}">

        <div class="tags has-addons">
            <a class="tag" href="{{ tag.tag.local_path }}">
                {{ tag.tag.name }}
            </a>
            {% if tag.tag.identifier in user_tags %}
            <button class="tag is-delete" type="submit">
                <span class="is-sr-only">remove tag</span>
            </button>
            {% else %}
            <button class="tag" type="submit">+
                <span class="is-sr-only">add tag</span>
            </button>
            {% endif %}
        </div>
    </form>
</div>