What it means
Fallbacks keep text usable while a webfont loads, after a failed request or when a particular symbol is missing. A font stack lists alternatives in order and normally ends with a generic family. Different devices may resolve that generic choice to different installed fonts.
Use it in practice
Choose alternatives with compatible proportions and test headings, buttons and paragraphs with the preferred font disabled. Check for layout shifts when it arrives.
Watch out for
Do not assume system-ui names one identical font on every device. A fallback that is much wider can change line breaks and button widths.
