Disallow not vertical rhythmed line-height.
Sources:
The following pattern is considered a violation:
.foo {
line-height: 12px;
}.foo {
line-height: 1.3;
}.foo {
line-height: 50px;
}The following patterns are not considered violations:
.foo {
line-height: 24px;
}.foo {
line-height: 1.6;
}.foo {
line-height: 48px;
}