Added div wrapper with role and label to the back to top button.

Added a div wrapper with role set to navigation, and an aria label (in case there are other elements with the navigation role already present)

js/back_to_top.js

@@ -3,8 +3,9 @@
     attach: function (context, settings) {
       var exist = $('#backtotop').length;
       if (exist == 0) {
+        let aria_label = Drupal.t("Back to top");
         $("body", context).once('backtotop').each(function () {
-          $('body').append("<button id='backtotop'>" + settings.back_to_top.back_to_top_button_text + "</button>");
+          $('body').append("<div role='navigation' aria-label='" + aria_label + "'><button id='backtotop' aria-label='" + aria_label + "'>" + settings.back_to_top.back_to_top_button_text + "</button></div>");
         });
       }