appendRule return value: undefined
fooRule.cssRules.length after appendRule: 2
appended rule: [object CSSKeyframeRule] ~ 25%, 75% { opacity: 0.5; }
appended rule parentRule === fooRule: true
fooRule.cssRules.length after duplicate appendRule: 3
original rule after duplicate appendRule: 0% { opacity: 0; }
duplicate rule: 0% { opacity: 1; }
fooRule.cssRules.length after invalid selector: 3
fooRule.cssRules.length after missing comma: 3
fooRule.cssRules.length after trailing comma: 3
fooRule.cssRules.length after multiple rules: 3
fooRule.cssRules.length after at-rule: 3
fooRule.cssRules.length after empty input: 3
