blog/source/_includes/post/flattr_button.html

<a class="FlattrButton" style="display:none;"
    title="{{ page.title }}"
    data-flattr-uid="{{ site.flattr_user }}"
    data-flattr-tags="{{ page.categories | join: "," }}"
    data-flattr-button="compact"
    data-flattr-category="text"
    href="{{ site.url }}{{ page.url }}">
    {% if page.description %}{{ page.description }}{% else %}{{page.content | truncate: 500}}{% endif %}
</a>