diff --git a/app/javascript/styles/mastodon/components.scss b/app/javascript/styles/mastodon/components.scss
index 8d0367889..dd49068e7 100644
--- a/app/javascript/styles/mastodon/components.scss
+++ b/app/javascript/styles/mastodon/components.scss
@@ -718,6 +718,10 @@
       font-family: inherit;
     }
 
+    u {
+      text-decoration: none;
+    }
+
     h1::before {
       content: '# ';
     }
@@ -788,6 +792,11 @@
     i {
       font-style: italic;
     }
+
+    sub {
+      font-size: smaller;
+      text-align: sub;
+    }
   }
 
   &.rich-blocks {
diff --git a/app/lib/sanitize_config.rb b/app/lib/sanitize_config.rb
index 0a87bf6df..23d0a418f 100644
--- a/app/lib/sanitize_config.rb
+++ b/app/lib/sanitize_config.rb
@@ -20,7 +20,7 @@ class Sanitize
     end
 
     MASTODON_STRICT ||= freeze_config(
-      elements: %w(p br span a abbr del pre blockquote code b strong i em h1 h2 h3 h4 h5 ul ol li),
+      elements: %w(p br span a abbr del pre blockquote code b strong u sub i em h1 h2 h3 h4 h5 ul ol li),
 
       attributes: {
         'a'          => %w(href rel class title),