Use aria labelled by on blocks to help with association.

templates/block.html.twig

@@ -0,0 +1,55 @@
+{#
+/**
+ * @file
+ * Theme override to display a block.
+ *
+ * Available variables:
+ * - plugin_id: The ID of the block implementation.
+ * - label: The configured label of the block if visible.
+ * - configuration: A list of the block's configuration values.
+ *   - label: The configured label for the block.
+ *   - label_display: The display settings for the label.
+ *   - provider: The module or other provider that provided this block plugin.
+ *   - Block plugin specific settings will also be stored here.
+ * - content: The content of this block.
+ * - attributes: array of HTML attributes populated by modules, intended to
+ *   be added to the main container tag of this template.
+ *   - id: A valid HTML ID and guaranteed unique.
+ * - title_attributes: Same as attributes, except applied to the main title
+ *   tag that appears in the template.
+ * - title_prefix: Additional output populated by modules, intended to be
+ *   displayed in front of the main title tag that appears in the template.
+ * - title_suffix: Additional output populated by modules, intended to be
+ *   displayed after the main title tag that appears in the template.
+ *
+ * @see template_preprocess_block()
+ */
+#}
+{% set heading_id = attributes.id ~ '-heading'|clean_id %}
+{% set cta_id     = attributes.id ~ '-cta'|clean_id %}
+
+<section{{ attributes }}>
+  {{ title_prefix }}
+  {% if label %}
+    <h2{{ title_attributes.setAttribute('id', heading_id) }}>{{ label }}</h2>
+  {% endif %}
+  {{ title_suffix }}
+
+  <div{{ content_attributes }}>
+  {% block content %}
+    {{ content|without('field_call_to_action') }}
+  {% endblock %}
+  </div>
+
+  {% if content.field_call_to_action %}{% spaceless %}
+  <a
+    id="{{ cta_id }}"
+    class="btn btn-default"
+    href="{{ content.field_call_to_action[0]['#url'] }}"
+    aria-labelledby="{{ cta_id }} {{ heading_id }}">
+
+    {{ content.field_call_to_action[0]['#title'] }}
+  </a>
+  {% endspaceless %}{% endif %}
+
+</section>