diff --git a/layout/style/nsCSSProps.cpp b/layout/style/nsCSSProps.cpp
index 73a8d5dd29..75a81aba00 100644
--- a/layout/style/nsCSSProps.cpp
+++ b/layout/style/nsCSSProps.cpp
@@ -2216,6 +2216,7 @@ const KTableEntry nsCSSProps::kUserSelectKTable[] = {
   { eCSSKeyword_auto,       StyleUserSelect::Auto },
   { eCSSKeyword_text,       StyleUserSelect::Text },
   { eCSSKeyword_element,    StyleUserSelect::Element },
+  { eCSSKeyword_contain,    StyleUserSelect::Contain },
   { eCSSKeyword_elements,   StyleUserSelect::Elements },
   { eCSSKeyword_all,        StyleUserSelect::All },
   { eCSSKeyword_toggle,     StyleUserSelect::Toggle },
diff --git a/layout/style/nsStyleConsts.h b/layout/style/nsStyleConsts.h
index 9507ea6fab..a708a6ff90 100644
--- a/layout/style/nsStyleConsts.h
+++ b/layout/style/nsStyleConsts.h
@@ -186,6 +186,7 @@ enum class StyleUserSelect : uint8_t {
   None,
   Text,
   Element,
+  Contain,
   Elements,
   All,
   Toggle,