diff --git a/plugins/addClassesToSVGElement.js b/plugins/addClassesToSVGElement.js
index 7f38dd0d..140223be 100644
--- a/plugins/addClassesToSVGElement.js
+++ b/plugins/addClassesToSVGElement.js
@@ -69,8 +69,17 @@ exports.fn = (root, params) => {
     element: {
       enter: (node, parentNode) => {
         if (node.name === 'svg' && parentNode.type === 'root') {
-          // @ts-ignore class attribute will be just a string eventually
-          node.class.add(...classNames);
+          const classList = new Set(
+            node.attributes.class == null
+              ? null
+              : node.attributes.class.split(' ')
+          );
+          for (const className of classNames) {
+            if (className != null) {
+              classList.add(className);
+            }
+          }
+          node.attributes.class = Array.from(classList).join(' ');
         }
       },
     },
diff --git a/plugins/inlineStyles.js b/plugins/inlineStyles.js
index 1d9d3de3..7e2c755d 100644
--- a/plugins/inlineStyles.js
+++ b/plugins/inlineStyles.js
@@ -207,13 +207,19 @@ exports.fn = function (root, opts) {
 
     for (selectedEl of selector.selectedEls) {
       // class
-      var firstSubSelector = selector.item.data.children.first();
+      const classList = new Set(
+        selectedEl.attributes.class == null
+          ? null
+          : selectedEl.attributes.class.split(' ')
+      );
+      const firstSubSelector = selector.item.data.children.first();
       if (firstSubSelector.type === 'ClassSelector') {
-        selectedEl.class.remove(firstSubSelector.name);
+        classList.delete(firstSubSelector.name);
       }
-      // clean up now empty class attributes
-      if (typeof selectedEl.class.item(0) === 'undefined') {
+      if (classList.size === 0) {
         delete selectedEl.attributes.class;
+      } else {
+        selectedEl.attributes.class = Array.from(classList).join(' ');
       }
 
       // ID
diff --git a/plugins/reusePaths.js b/plugins/reusePaths.js
index 0d9c1fa7..43b7f069 100644
--- a/plugins/reusePaths.js
+++ b/plugins/reusePaths.js
@@ -62,16 +62,7 @@ exports.fn = function (root) {
     );
     root.children[0].spliceContent(0, 0, defsTag);
     for (let def of defs) {
-      // Remove class and style before copying to avoid circular refs in
-      // JSON.stringify. This is fine because we don't actually want class or
-      // style information to be copied.
-      const style = def.style;
-      const defClass = def.class;
-      delete def.style;
-      delete def.class;
       const defClone = def.clone();
-      def.style = style;
-      def.class = defClass;
       delete defClone.attributes.transform;
       defsTag.spliceContent(0, 0, defClone);
       // Convert the original def to a use so the first usage isn't duplicated.
diff --git a/test/plugins/addClassesToSVGElement.03.svg b/test/plugins/addClassesToSVGElement.03.svg
new file mode 100644
index 00000000..a9ba2cea
--- /dev/null
+++ b/test/plugins/addClassesToSVGElement.03.svg
@@ -0,0 +1,17 @@
+Should avoid adding existing classes
+
+===
+
+<svg xmlns="http://www.w3.org/2000/svg" class="mySvg">
+    test
+</svg>
+
+@@@
+
+<svg xmlns="http://www.w3.org/2000/svg" class="mySvg size-big">
+    test
+</svg>
+
+@@@
+
+{"classNames":["mySvg","size-big"]}