diff --git a/cjs/html/document.js b/cjs/html/document.js
index 85f1a4f8..1274dd98 100644
--- a/cjs/html/document.js
+++ b/cjs/html/document.js
@@ -10,9 +10,10 @@ const {customElements} = require('../interface/custom-element-registry.js');
 const {HTMLElement} = require('./element.js');
 
 const createHTMLElement = (ownerDocument, builtin, localName, options) => {
-  if (!builtin && htmlClasses.has(localName)) {
+  if (!builtin) {
     const Class = htmlClasses.get(localName);
-    return new Class(ownerDocument, localName);
+    if (Class)
+      return new Class(ownerDocument, localName);
   }
   const {[CUSTOM_ELEMENTS]: {active, registry}} = ownerDocument;
   if (active) {
diff --git a/cjs/shared/parse-from-string.js b/cjs/shared/parse-from-string.js
index 84a9fdf3..f85f205c 100644
--- a/cjs/shared/parse-from-string.js
+++ b/cjs/shared/parse-from-string.js
@@ -27,12 +27,23 @@ const append = (self, node, active) => {
   return node;
 };
 
+/** Same tokens as a DOMTokenList would hold: no empty entries, no duplicates, single spaces. */
+const classTokens = value => {
+  const tokens = [];
+  for (const token of value.split(/\s+/)) {
+    if (token && !tokens.includes(token))
+      tokens.push(token);
+  }
+  return tokens.join(' ');
+};
+
 const attribute = (element, end, attribute, value, active) => {
   attribute[VALUE] = value;
   attribute.ownerElement = element;
   knownSiblings(end[PREV], attribute, end);
+  // the token list is built on demand: while it does not exist, only the value needs the tokens
   if (attribute.name === 'class')
-    element.className = value;
+    attribute[VALUE] = classTokens(value);
   if (active)
     attributeChangedCallback(element, attribute.name, null, value);
 };
diff --git a/esm/html/document.js b/esm/html/document.js
index 47e23bca..ea6a9a3c 100644
--- a/esm/html/document.js
+++ b/esm/html/document.js
@@ -9,9 +9,10 @@ import {customElements} from '../interface/custom-element-registry.js';
 import {HTMLElement} from './element.js';
 
 const createHTMLElement = (ownerDocument, builtin, localName, options) => {
-  if (!builtin && htmlClasses.has(localName)) {
+  if (!builtin) {
     const Class = htmlClasses.get(localName);
-    return new Class(ownerDocument, localName);
+    if (Class)
+      return new Class(ownerDocument, localName);
   }
   const {[CUSTOM_ELEMENTS]: {active, registry}} = ownerDocument;
   if (active) {
diff --git a/esm/shared/parse-from-string.js b/esm/shared/parse-from-string.js
index 08c6fe39..af30aac2 100644
--- a/esm/shared/parse-from-string.js
+++ b/esm/shared/parse-from-string.js
@@ -26,12 +26,23 @@ const append = (self, node, active) => {
   return node;
 };
 
+/** Same tokens as a DOMTokenList would hold: no empty entries, no duplicates, single spaces. */
+const classTokens = value => {
+  const tokens = [];
+  for (const token of value.split(/\s+/)) {
+    if (token && !tokens.includes(token))
+      tokens.push(token);
+  }
+  return tokens.join(' ');
+};
+
 const attribute = (element, end, attribute, value, active) => {
   attribute[VALUE] = value;
   attribute.ownerElement = element;
   knownSiblings(end[PREV], attribute, end);
+  // the token list is built on demand: while it does not exist, only the value needs the tokens
   if (attribute.name === 'class')
-    element.className = value;
+    attribute[VALUE] = classTokens(value);
   if (active)
     attributeChangedCallback(element, attribute.name, null, value);
 };
diff --git a/test/html/element.js b/test/html/element.js
index 3c3fedd8..88107f3f 100644
--- a/test/html/element.js
+++ b/test/html/element.js
@@ -159,3 +159,18 @@ assert(node.innerHTML, '<video src="" controls></video>');
 node.innerHTML = '<div>The <strong>quick</strong> brown fox</div><div>Jumped over<br>The lazy\ndog</div>';
 assert(node.innerText, 'The quick brown fox\nJumped over\nThe lazy dog', 'innerText newlines');
 assert(node.textContent, 'The quick brown foxJumped overThe lazy\ndog', 'textContent no newlines');
+
+// a parsed class attribute keeps every character of its tokens, drops empty and duplicate tokens,
+// and is escaped only when it is serialized
+{
+  const source = '<!DOCTYPE html><html><body><div class="  a<b  &amp;c  a<b  d&quot;e "></div></body></html>';
+  const {document} = parseHTML(source);
+  const div = document.querySelector('div');
+  assert(div.getAttribute('class'), 'a<b &c d"e', 'class value with markup characters');
+  assert(div.className, 'a<b &c d"e', 'className with markup characters');
+  assert(div.classList.length, 3, 'empty and duplicate class tokens dropped');
+  assert([...div.classList].join('|'), 'a<b|&c|d"e', 'class tokens with markup characters');
+  assert(div.outerHTML, '<div class="a<b &c d&quot;e"></div>', 'only the quote is escaped in html');
+  const {document: reparsed} = parseHTML(`<!DOCTYPE html><html><body>${div.outerHTML}</body></html>`);
+  assert(reparsed.querySelector('div').getAttribute('class'), 'a<b &c d"e', 'class value round trip');
+}
diff --git a/test/xml/document.js b/test/xml/document.js
index deaa763c..59a01c6f 100644
--- a/test/xml/document.js
+++ b/test/xml/document.js
@@ -19,3 +19,10 @@ document.documentElement.innerHTML = `
 
 assert(document.querySelectorAll('Element').length, 2, 'case sesntivive 2');
 assert(document.querySelectorAll('element').length, 0, 'case sesntivive 0');
+
+// in xml every character of a class value is escaped on output, as for any other attribute
+{
+  const xml = (new DOMParser).parseFromString('<root class="a&lt;b  &amp;c  a&lt;b"/>', 'text/xml');
+  assert(xml.documentElement.getAttribute('class'), 'a<b &c', 'class value with markup characters');
+  assert(xml.documentElement.toString(), '<root class="a&lt;b &amp;c" />', 'class value escaped in xml');
+}