FIX: Also support just `open`

app/assets/javascripts/pretty-text/sanitizer.js.es6

@@ -3,7 +3,11 @@ import xss from 'pretty-text/xss';
 const _validIframes = [];
 
 function attr(name, value) {
-  return `${name}="${xss.escapeAttrValue(value)}"`;
+  if (value) {
+    return `${name}="${xss.escapeAttrValue(value)}"`;
+  }
+
+  return name;
 }
 
 const ESCAPE_REPLACEMENTS = {

plugins/discourse-details/assets/javascripts/lib/discourse-markdown/details.js.es6

@@ -25,7 +25,7 @@ export function setup(helper) {
     'summary',
     'summary[title]',
     'details',
-    'details[open=open]',
+    'details[open]',
     'details.elided'
   ]);
 

plugins/discourse-details/test/javascripts/lib/details-cooked-test.js.es6

@@ -28,4 +28,8 @@ test("details", () => {
   cooked(`<details open="open"><summary>Info</summary>coucou</details>`,
          `<details open="open"><summary>Info</summary>\n\n<p>coucou</p>\n\n</details>`,
          "open attribute");
+
+  cooked(`<details open><summary>Info</summary>coucou</details>`,
+         `<details open><summary>Info</summary>\n\n<p>coucou</p>\n\n</details>`,
+         "open attribute");
 });