relativeToAbsoluteValue = function(value, elem, scrollTop = getScrollTop(), viewportHeight = getViewportHeight()) {

	const elemSize = elem.getBoundingClientRect()

	const elemAnchor = value.match(/^[a-z]+/)[0]
	const viewportAnchor = value.match(/[a-z]+$/)[0]

	let y = 0

	if (viewportAnchor === 'top') y -= 0
	if (viewportAnchor === 'middle') y -= viewportHeight / 2
	if (viewportAnchor === 'bottom') y -= viewportHeight

	if (elemAnchor === 'top') y += (elemSize.top + scrollTop)
	if (elemAnchor === 'middle') y += (elemSize.top + scrollTop) + elemSize.height / 2
	if (elemAnchor === 'bottom') y += (elemSize.top + scrollTop) + elemSize.height

	return `${ y }px`

}
Converts a relative value to an absolute value.
@param {String} value
@param {Node} elem - Anchor of the relative value.
@param {?Integer} scrollTop - Pixels scrolled in document.
@param {?Integer} viewportHeight - Height of the viewport.
@returns {String} value - Absolute value.