Installing Khmer fonts on a Mac is straightforward since macOS actually comes with native support, though you might want extra "Unicode" fonts for specific design needs. 1. Check Pre-installed Fonts
Many users prefer the traditional NiDA layout used in Cambodia. Since macOS doesn't include it by default, you can install third-party packages: socheatsok78/khmer-unicode-keyboard-for-macos - GitHub
Solution: Safari is more restrictive about font rendering. Go to Safari > Settings > Advanced and ensure "Never use font sizes smaller than..." is unchecked. Also, in Font Book, validate the font – Safari may reject a font with minor validation errors.
Click the + (Plus) button at the top or drag and drop your downloaded font files into the window.
Cmd + O).