The markerHeight attribute represents the height of the viewport into which the <marker> is to be fitted when it is rendered according to the viewBox and preserveAspectRatio attributes.
You can use this attribute with the following SVG elements:
This value defines either an absolute or a relative height of the marker. Relative values refer to the height specified via the viewBox and preserveAspectRatio attributes.
<number>
This value defines the height of the marker in the units defined by the markerUnits attribute.
A value of zero disables rendering of the element and negative values are an error.