Updating font size in Safari to 36px make h1 element smaller than it was. Decrease from 48px to 36px.
The possible issue that parent elements using a combination of em and rem units for font size, what leads Safari to inherit not correct value.
Instructions how set font size on MacOs:
Preferences > Advanced > Accessibility: set the font size to 36px (which is 200% of the standard 16px)

