diff --git a/sphinx/writers/html5.py b/sphinx/writers/html5.py
index 0f96d1bdf..bba07f447 100644
--- a/sphinx/writers/html5.py
+++ b/sphinx/writers/html5.py
@@ -13,7 +13,7 @@ import posixpath
 import re
 import urllib.parse
 import warnings
-from typing import TYPE_CHECKING, Iterable, Tuple, cast
+from typing import TYPE_CHECKING, Iterable, Set, Tuple, cast
 
 from docutils import nodes
 from docutils.nodes import Element, Node, Text
@@ -59,7 +59,7 @@ class HTML5Translator(SphinxTranslator, BaseTranslator):
     # Override docutils.writers.html5_polyglot:HTMLTranslator
     # otherwise, nodes like <inline classes="s">...</inline> will be
     # converted to <s>...</s> by `visit_inline`.
-    supported_inline_tags = set()
+    supported_inline_tags: Set[str] = set()
 
     def __init__(self, document: nodes.document, builder: Builder) -> None:
         super().__init__(document, builder)