The width read-only property of the SVGFESpecularLightingElement interface describes the horizontal size of an SVG filter primitive as a SVGAnimatedLength.
It reflects the width attribute of the <feSpecularLighting> element, which lights a source graphic using the alpha channel as a bump map. The attribute is a <length> or a <percentage> relative to the width of the filter region. The default value is 100%. The property value is a length in user coordinate system units.
const feSpecularLighting = document.querySelector("feSpecularLighting");
const horizontalSize = feSpecularLighting.width;
console.log(horizontalSize.baseVal.value); // the `width` value