Start with default generated template

templates/block/block--local-actions-block.html.twig

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+{% extends "block.html.twig" %}
+{#
+/**
+ * @file
+ * Theme override for local actions (primary admin actions.)
+ */
+#}
+{% block content %}
+  {% if content %}
+    <nav class="action-links">{{ content }}</nav>
+  {% endif %}
+{% endblock %}

templates/block/block--local-tasks-block.html.twig

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+{% extends "block.html.twig" %}
+{#
+/**
+ * @file
+ * Theme override for tabs.
+ */
+#}
+{% block content %}
+  {% if content %}
+    <nav class="tabs" role="navigation" aria-label="{{ 'Tabs'|t }}">
+      {{ content }}
+    </nav>
+  {% endif %}
+{% endblock %}

templates/block/block--search-form-block.html.twig

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+{#
+/**
+ * @file
+ * Theme override for the search form 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, including:
+ *   - 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.
+ * - in_preview: Whether the plugin is being rendered in preview mode.
+ * - content: The content of this block.
+ * - attributes: A list HTML attributes populated by modules, intended to
+ *   be added to the main container tag of this template. Includes:
+ *   - 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()
+ * @see search_preprocess_block()
+ */
+#}
+{%
+  set classes = [
+    'block',
+    'block-search',
+    'container-inline',
+  ]
+%}
+<div{{ attributes.addClass(classes) }}>
+  {{ title_prefix }}
+  {% if label %}
+    <h2{{ title_attributes }}>{{ label }}</h2>
+  {% endif %}
+  {{ title_suffix }}
+  {% block content %}
+    {{ content }}
+  {% endblock %}
+</div>

templates/block/block--system-branding-block.html.twig

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+{% extends "block.html.twig" %}
+{#
+/**
+ * @file
+ * Theme override for a branding block.
+ *
+ * Each branding element variable (logo, name, slogan) is only available if
+ * enabled in the block configuration.
+ *
+ * Available variables:
+ * - site_logo: Logo for site as defined in Appearance or theme settings.
+ * - site_name: Name for site as defined in Site information settings.
+ * - site_slogan: Slogan for site as defined in Site information settings.
+ */
+#}
+{% block content %}
+  {% if site_logo %}
+    <a href="{{ path('<front>') }}" rel="home" class="site-logo">
+      <img src="{{ site_logo }}" alt="{{ 'Home'|t }}" fetchpriority="high" />
+    </a>
+  {% endif %}
+  {% if site_name %}
+    <div class="site-name">
+      <a href="{{ path('<front>') }}" rel="home">{{ site_name }}</a>
+    </div>
+  {% endif %}
+  {% if site_slogan %}
+    <div class="site-slogan">{{ site_slogan }}</div>
+  {% endif %}
+{% endblock %}

templates/block/block--system-menu-block.html.twig

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+{#
+/**
+ * @file
+ * Theme override for a menu 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.
+ * - in_preview: Whether the plugin is being rendered in preview mode.
+ * - content: The content of this block.
+ * - attributes: HTML attributes for the containing element.
+ *   - id: A valid HTML ID and guaranteed unique.
+ * - title_attributes: HTML attributes for the title element.
+ * - content_attributes: HTML attributes for the content element.
+ * - 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.
+ *
+ * Headings should be used on navigation menus that consistently appear on
+ * multiple pages. When this menu block's label is configured to not be
+ * displayed, it is automatically made invisible using the 'visually-hidden' CSS
+ * class, which still keeps it visible for screen-readers and assistive
+ * technology. Headings allow screen-reader and keyboard only users to navigate
+ * to or skip the links.
+ * See http://juicystudio.com/article/screen-readers-display-none.php and
+ * https://www.w3.org/TR/WCAG-TECHS/H42.html for more information.
+ */
+#}
+{%
+  set classes = [
+    'block',
+    'block-menu',
+    'navigation',
+    'menu--' ~ derivative_plugin_id|clean_class,
+  ]
+%}
+{% set heading_id = attributes.id ~ '-menu'|clean_id %}
+<nav role="navigation" aria-labelledby="{{ heading_id }}"{{ attributes.addClass(classes)|without('role', 'aria-labelledby') }}>
+  {# Label. If not displayed, we still provide it for screen readers. #}
+  {% if not configuration.label_display %}
+    {% set title_attributes = title_attributes.addClass('visually-hidden') %}
+  {% endif %}
+  {{ title_prefix }}
+  <h2{{ title_attributes.setAttribute('id', heading_id) }}>{{ configuration.label }}</h2>
+  {{ title_suffix }}
+
+  {# Menu. #}
+  {% block content %}
+    {{ content }}
+  {% endblock %}
+</nav>

templates/block/block.html.twig

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+{#
+/**
+ * @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.
+ * - in_preview: Whether the plugin is being rendered in preview mode.
+ * - 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 classes = [
+    'block',
+    'block-' ~ configuration.provider|clean_class,
+    'block-' ~ plugin_id|clean_class,
+  ]
+%}
+<div{{ attributes.addClass(classes) }}>
+  {{ title_prefix }}
+  {% if label %}
+    <h2{{ title_attributes }}>{{ label }}</h2>
+  {% endif %}
+  {{ title_suffix }}
+  {% block content %}
+    {{ content }}
+  {% endblock %}
+</div>