function transform(node, attr) {
		const { tag, attrs } = node;
		const url = attrs && attrs[attr];
		if (typeof tag === 'string' && url) {
			const out = transformUrl(url, attr, tag, node);
			if (out != null && out !== false) {
				attrs[attr] = out;
				cache.set(url, out);
			}
		}
	}
@param {posthtml.Node} node, @param {string} attr