CSS.registerProperty({
  name: '--animation-duration',
  syntax: '<time>',
  inherits: false,
  initialValue: '1s'
});

.my-el {
  --animation-duration: 1.5s;
}

const duration = CSSNumericValue.parse(getComputedStyle(el).getPropertyValue('--ripple-anim-duration')).to('ms').value; // 1500

const duration = properties.get('--ripple-anim-duration').to('ms').value; // 1500

const duration = el.attributeStyleMap.get('--ripple-anim-duration').to('ms').value; // ¯_(ツ)_/¯