code: function getPropertyValue(
    cssRule: HTMLElementWithStyleMap | CSSStyleRule | CSSKeyframeRule,
    prop: string
  ): string {
    try {
      // Support CSSTOM.
      if (cssRule.attributeStyleMap) {
        return cssRule.attributeStyleMap.get(prop)
      }
      return cssRule.style.getPropertyValue(prop)
    } catch (err) {
      // IE may throw if property is unknown.
      return ''
    }
  }
docs: Get a style property value.