Remove the div around block content - and make sure this doesn't break any CSS defined in core modules.
Current block code:
<div class="block {{ attributes.class }}"{{ attributes }}>
{{ title_prefix }}
{% if label %}
<h2{{ title_attributes }}>{{ label }}</h2>
{% endif %}
{{ title_suffix }}
<div class="content {{ content_attributes.class }}"{{ content_attributes }}>
{{ content }}
</div>
</div>Proposed change:
<div class="block {{ attributes.class }}"{{ attributes }}>
{{ title_prefix }}
{% if label %}
<h2{{ title_attributes }}>{{ label }}</h2>
{% endif %}
{{ title_suffix }}
{{ content }}
</div>