root: add px to the disallow list for css units

Change-Id: Idcbd499d5b9cbc77047fb5a945d205ff0feb4a03
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/281773
Trust: Jamal Carvalho <jamal@golang.org>
Run-TryBot: Jamal Carvalho <jamal@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
diff --git a/.stylelintrc.yaml b/.stylelintrc.yaml
index 46005f4..e917845 100644
--- a/.stylelintrc.yaml
+++ b/.stylelintrc.yaml
@@ -1,3 +1,6 @@
 extends:
   - stylelint-config-recommended
   - stylelint-prettier/recommended
+rules:
+  unit-disallowed-list:
+    - "px"