diff --git a/src/scss/global.scss b/src/scss/global.scss
index abc54481..9e14d2dd 100644
--- a/src/scss/global.scss
+++ b/src/scss/global.scss
@@ -21,6 +21,13 @@ body {
 .main-content {
   contain: content; // see https://www.w3.org/TR/2018/CR-css-contain-1-20181108/#valdef-contain-content
   padding-top: var(--main-content-pad-top);
+
+  @supports not (contain: content) {
+    // For browsers which don't support the "contain" CSS property,
+    // make a stacking context manually so that the z-index behavior
+    // is consistent. See: https://github.com/nolanlawson/pinafore/issues/1806
+    isolation: isolate;
+  }
 }
 
 main {