Fix removing attributes from DOM element.

packages/ckeditor5-engine/src/view/renderer.ts

@@ -590,7 +590,9 @@ export default class Renderer extends /* #__PURE__ */ ObservableMixin() {
 		// Note: It is important to first remove DOM attributes and then set new ones, because some view attributes may be renamed
 		// as they are set on DOM (due to unsafe attributes handling). If we set the view attribute first, and then remove
 		// non-existing DOM attributes, then we would remove the attribute that we just set.
-		for ( const domAttr of ( domElement as DomElement ).attributes ) {
+		//
+		// Note: The domElement.attributes is a live collection, so we need to convert it to an array to avoid issues.
+		for ( const domAttr of Array.from( ( domElement as DomElement ).attributes ) ) {
 			const key = domAttr.name;
 
 			// All other attributes not present in the DOM should be removed.

packages/ckeditor5-engine/tests/view/renderer.js

@@ -189,6 +189,19 @@ describe( 'Renderer', () => {
 			expect( renderer.markedAttributes.size ).to.equal( 0 );
 		} );
 
+		it( 'should remove all attributes', () => {
+			domRoot.setAttribute( 'style', 'border:1px solid red' );
+			domRoot.setAttribute( 'class', 'bar' );
+
+			renderer.markToSync( 'attributes', viewRoot );
+			renderer.render();
+
+			expect( domRoot.getAttribute( 'class' ) ).to.be.not.ok;
+			expect( domRoot.getAttribute( 'style' ) ).to.be.not.ok;
+
+			expect( renderer.markedAttributes.size ).to.equal( 0 );
+		} );
+
 		it( 'should add children', () => {
 			viewRoot._appendChild( new ViewText( viewDocument, 'foo' ) );