Add more information about rem units and padding

source/manuals/programming-languages/css.html.md.erb

@@ -31,11 +31,15 @@ You should use the [GOV.UK Design System spacing scale](https://design-system.se
 
 If your project is not using the GOV.UK Design System, you should use pixel values in multiples of `5px`.
 
-### Use `rem` units for font size, but not for spacing
+## Use `rem` units strategically
 
-Use `rem` units for font sizing wherever possible. This allows users to increase the size of the text on a page by adjusting the font size in their browser settings.
+As a general rule, per [Josh W. Comeau's _The Surprising Truth About Pixels and Accessibility_] (https://www.joshwcomeau.com/css/surprising-truth-about-pixels-and-accessibility/#strategic-unit-deployment-6):
 
-Avoid using `rem` units for spacing - use `px` instead.
+> When picking between pixels and rems, here's the question you should be asking yourself: Should this value scale up as the user increases their browser's default font size?
+
+Use `rem` units for `font-size` wherever possible.
+
+You should generally avoid using `rem` for padding or border-width if these are likely to cause layout or readability issues when the font-size increases.
 
 ## Vendor prefixing