diff --git a/.github/workflows/compatibility.yaml b/.github/workflows/compatibility.yaml
index f2afb889..822da8e0 100644
--- a/.github/workflows/compatibility.yaml
+++ b/.github/workflows/compatibility.yaml
@@ -38,6 +38,14 @@ jobs:
       - name: Validate composer.json
         run: composer validate --strict
 
+      # Contao 4.13 pulls in spomky-labs/otphp ^10.0 transitively, and every
+      # available 10.x release carries a security advisory. Composer refuses to
+      # resolve advisory-affected packages by default, which breaks this
+      # dry-run resolvability check for a transitive constraint we don't control.
+      # Disable the advisory block for this ephemeral CI checkout only.
+      - name: Allow advisory-affected transitive dependencies
+        run: composer config --no-plugins policy.advisories.block false
+
       - name: Check dependency resolution
         run: >-
           composer update --dry-run --no-progress --prefer-dist
diff --git a/composer.json b/composer.json
index 08694a95..71985cc8 100644
--- a/composer.json
+++ b/composer.json
@@ -25,7 +25,7 @@
         "symfony/serializer": "^5.4 || ^6.0 || ^7.0",
         "symfony/string": "^5.2 || ^6.0 || ^7.0",
         "symfony/validator": "^5.4 || ^6.0 || ^7.0",
-        "twig/twig": "^3.0"
+        "twig/twig": "^3.13"
     },
     "require-dev": {
         "contao/test-case": "^4.0 || ^5.0",
diff --git a/contao/templates/content_element/flare_listview.html.twig b/contao/templates/content_element/flare_listview.html.twig
index 03dda1d2..a0793c3c 100644
--- a/contao/templates/content_element/flare_listview.html.twig
+++ b/contao/templates/content_element/flare_listview.html.twig
@@ -1,6 +1,8 @@
 {# @var \HeimrichHannot\FlareBundle\Engine\Engine flare #}
 {% extends '@Contao/flare/_flare_base.html.twig' %}
 
+{% set flare_css_classes = [flare_css_classes ?? '', 'flare-listview', 'content-flare-listview'] %}
+
 {# @var \HeimrichHannot\FlareBundle\Engine\View\InteractiveView flare_list #}
 {% set flare_list = flare.createView %}
 
diff --git a/contao/templates/content_element/flare_reader.html.twig b/contao/templates/content_element/flare_reader.html.twig
index aad60f8d..1492af61 100644
--- a/contao/templates/content_element/flare_reader.html.twig
+++ b/contao/templates/content_element/flare_reader.html.twig
@@ -3,6 +3,8 @@
 {# @var \Contao\Model model #}
 {% extends '@Contao/flare/_flare_base.html.twig' %}
 
+{% set flare_css_classes = [flare_css_classes ?? '', 'flare-reader', 'content-flare-reader'] %}
+
 {% block content %}
 
     <div style="color: #800; background: #fdd; padding: 1rem;">
diff --git a/contao/templates/flare/_flare_base.html.twig b/contao/templates/flare/_flare_base.html.twig
index f01b7d5b..f8b69ae5 100644
--- a/contao/templates/flare/_flare_base.html.twig
+++ b/contao/templates/flare/_flare_base.html.twig
@@ -1,4 +1,11 @@
-{% set html_class = ('flare-listview content-flare-listview block ' ~ element_css_classes|default(class|default('')) ~ ' ' ~ flare_css_classes|default('')) %}
+{% types {
+    element_css_classes?: 'string',
+    class?: 'string',
+    flare_css_classes?: 'string|string[]',
+    html_class: 'string',
+    html_id?: 'string',
+} %}
+{% set html_class = flare_str.htmlJoinClasses('block', element_css_classes ?? class ?? '', flare_css_classes ?? '') %}
 {% set html_id = element_html_id|default(null) %}
 <div class="{{ html_class|trim }}"{% if html_id is not null %} id="{{ html_id }}"{% elseif cssID|default %}{{ cssID|raw }}{% endif %}>
     {% block wrapper %}
diff --git a/src/Util/Str.php b/src/Util/Str.php
index 830b4901..2db295b2 100644
--- a/src/Util/Str.php
+++ b/src/Util/Str.php
@@ -139,7 +139,7 @@ public static function wrap(mixed $value): string
 
     public static function random(int $length = 10, ?string $chars = null): string
     {
-        $chars ??= '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
+        $chars ??= self::CHARS_ALPHANUMERIC;
         $max = \mb_strlen($chars, '8bit') - 1;
         $rand = '';
 
@@ -294,4 +294,39 @@ public static function coalesce(string|callable|null ...$args): ?string
 
         return null;
     }
+
+    public static function htmlListClasses(string|array|null ...$classes): array
+    {
+        if (!$classes) {
+            return [];
+        }
+
+        $normalClasses = [];
+        $flatClasses = Arr::flatten($classes);
+
+        foreach ($flatClasses as $class)
+        {
+            if ($class instanceof \Closure) {
+                $class = $class();
+            }
+
+            if (!$class) {
+                continue;
+            }
+
+            if (!\is_scalar($class) && !$class instanceof \Stringable) {
+                continue;
+            }
+
+            $split = \explode(' ', (string) $class);
+            \array_push($normalClasses, ...$split);
+        }
+
+        return \array_values(\array_unique(\array_filter($normalClasses)));
+    }
+
+    public static function htmlJoinClasses(string|array|null ...$classes): string
+    {
+        return \implode(' ', self::htmlListClasses(...$classes));
+    }
 }
\ No newline at end of file