require-system-font-fallback

Require font-family stacks to end with a system fallback.

Rule catalog ID: R015

Targeted pattern scope

• font-family declarations outside @font-face.

What this rule reports

• Stacks that do not end with a system fallback name such as system-ui.

Why this rule exists

Explicit system fallback improves resiliency when custom fonts fail.

❌ Incorrect

.title {
  font-family: "Inter", Arial;
}

✅ Correct

.title {
  font-family: "Inter", system-ui;
}

Further reading

• MDN: system-ui