diff --git a/.changeset/popular-wasps-compare.md b/.changeset/popular-wasps-compare.md
new file mode 100644
index 0000000000..eb2418d018
--- /dev/null
+++ b/.changeset/popular-wasps-compare.md
@@ -0,0 +1,5 @@
+---
+"@primer/css": patch
+---
+
+Add `font-size` to `html` tag to make rem units "safe"
diff --git a/src/base/normalize.scss b/src/base/normalize.scss
index b754c5f4e7..5621566de1 100644
--- a/src/base/normalize.scss
+++ b/src/base/normalize.scss
@@ -7,6 +7,7 @@
  */
 
 html {
+  font-size: 16px; /* this can be removed when all of GitHub uses rem units */
   font-family: sans-serif; /* 1 */
   -ms-text-size-adjust: 100%; /* 2 */
   -webkit-text-size-adjust: 100%; /* 2 */