* XMLSerializer instance used to convert DOM elements to an XML string.
* 
* @private
* @member {XMLSerializer}
*/
this._xmlSerializer = new XMLSerializer();

...

toData( viewFragment ) {
    // Convert view DocumentFragment to DOM DocumentFragment.
    const domFragment = this._domConverter.viewToDom( viewFragment, document );

    // Convert DOM DocumentFragment to XML output.
    const doc = document.implementation.createHTMLDocument( '' );
    const container = doc.createElement( 'div' );
    container.appendChild( domFragment );
    return this._xmlSerializer.serializeToString(container);
}

class CustomWriter {
	/**
	 * Returns an HTML string created from the document fragment.
	 *
	 * @param {DocumentFragment} fragment
	 * @returns {String}
	 */
	getHtml( fragment ) {
		const parser = new DOMParser();
		const doc = parser.parseFromString( '<root></root>', 'text/xml' );

		const container = doc.children[ 0 ];
		container.appendChild( fragment );

		return container.innerHTML;
	}
}

function customPlugin( editor ) {
	console.log( 'using a custom processor' );
	editor.data.processor = new XmlDataProcessor( editor.editing.view.document, { namespaces: [ 'foo' ] } );

	editor.data.processor._htmlWriter = new CustomWriter();
}

<h2 xmlns="http://www.w3.org/1999/xhtml">Lists in the table</h2>
<hr xmlns="http://www.w3.org/1999/xhtml" />

<p xmlns="http://www.w3.org/1999/xhtml">foodaskljd
	<br /><br /> </p>

<figure xmlns="http://www.w3.org/1999/xhtml" class="image">
	<img src="../assets/img/fields.jpg" alt="Autumn fields" />
</figure>