diff --git a/core/css.ts b/core/css.ts
index 503b6362ba2..37de8e761e1 100644
--- a/core/css.ts
+++ b/core/css.ts
@@ -59,6 +59,15 @@ export function inject(hasCss: boolean, pathToMedia: string) {
  * The CSS content for Blockly.
  */
 let content = `
+:is(
+  .injectionDiv,
+  .blocklyWidgetDiv,
+  .blocklyDropdownDiv,
+  .blocklyTooltipDiv,
+) * {
+  box-sizing: border-box;
+}
+
 .blocklySvg {
   background-color: #fff;
   outline: none;