diff --git a/web_src/js/features/comp/EditorMarkdown.test.ts b/web_src/js/features/comp/EditorMarkdown.test.ts
index 611dd32146a..7043698cda5 100644
--- a/web_src/js/features/comp/EditorMarkdown.test.ts
+++ b/web_src/js/features/comp/EditorMarkdown.test.ts
@@ -199,5 +199,5 @@ test('EditorMarkdown', () => {
   testInput('- [ ]', '- [ ]\n- ');
   testInput('- [ ] foo', '- [ ] foo\n- [ ] ');
   testInput('* [x] foo', '* [x] foo\n* [ ] ');
-  testInput('1. [x] foo', '1. [x] foo\n2. [ ] ');
+  testInput('1. [X] foo', '1. [X] foo\n2. [ ] ');
 });
diff --git a/web_src/js/features/comp/EditorMarkdown.ts b/web_src/js/features/comp/EditorMarkdown.ts
index 2ccc9de112b..540dfdbfb38 100644
--- a/web_src/js/features/comp/EditorMarkdown.ts
+++ b/web_src/js/features/comp/EditorMarkdown.ts
@@ -148,9 +148,9 @@ export function markdownHandleIndention(tvs: TextareaValueSelection): MarkdownHa
   lineContent = lineContent.slice(indention.length);
   if (linesBuf.inlinePos <= indention.length) return unhandled; // if cursor is at the indention, do nothing, let the browser handle it
 
-  // parse the prefixes: "1. ", "- ", "* ", there could also be " [ ] " or " [x] " for task lists
+  // parse the prefixes: "1. ", "- ", "* ", there could also be " [ ] ", " [x] " or " [X] " for task lists
   // there must be a space after the prefix because none of "1.foo" / "-foo" is a list item
-  const prefixMatch = /^([0-9]+\.|[-*])(\s\[([ x])\])?\s/.exec(lineContent);
+  const prefixMatch = /^([0-9]+\.|[-*])(\s\[([ xX])\])?\s/.exec(lineContent);
   let prefix = '';
   if (prefixMatch) {
     prefix = prefixMatch[0];
@@ -168,7 +168,7 @@ export function markdownHandleIndention(tvs: TextareaValueSelection): MarkdownHa
     // start a new line with the same indention
     let newPrefix = prefix;
     if (/^\d+\./.test(prefix)) newPrefix = `1. ${newPrefix.slice(newPrefix.indexOf('.') + 2)}`;
-    newPrefix = newPrefix.replace('[x]', '[ ]');
+    newPrefix = newPrefix.replace(/\[[xX]\]/, '[ ]');
 
     const inlinePos = linesBuf.inlinePos;
     linesBuf.lines[linesBuf.posLineIndex] = line.substring(0, inlinePos);
diff --git a/web_src/js/markup/tasklist.test.ts b/web_src/js/markup/tasklist.test.ts
index ec5eceebd07..788eb9afda2 100644
--- a/web_src/js/markup/tasklist.test.ts
+++ b/web_src/js/markup/tasklist.test.ts
@@ -3,6 +3,7 @@ import {toggleTasklistCheckbox} from './tasklist.ts';
 test('toggleTasklistCheckbox', () => {
   expect(toggleTasklistCheckbox('- [ ] task', 3, true)).toEqual('- [x] task');
   expect(toggleTasklistCheckbox('- [x] task', 3, false)).toEqual('- [ ] task');
+  expect(toggleTasklistCheckbox('- [X] task', 3, false)).toEqual('- [ ] task');
   expect(toggleTasklistCheckbox('- [ ] task', 0, true)).toBeNull();
   expect(toggleTasklistCheckbox('- [ ] task', 99, true)).toBeNull();
   expect(toggleTasklistCheckbox('😀 - [ ] task', 8, true)).toEqual('😀 - [x] task');
diff --git a/web_src/js/markup/tasklist.ts b/web_src/js/markup/tasklist.ts
index 557afeaea58..1cc5bd1ce64 100644
--- a/web_src/js/markup/tasklist.ts
+++ b/web_src/js/markup/tasklist.ts
@@ -5,14 +5,14 @@ const preventListener = (e: Event) => e.preventDefault();
 
 /**
  * Toggle a task list checkbox in markdown content.
- * `position` is the byte offset of the space or `x` character inside `[ ]`.
+ * `position` is the byte offset of the space, `x` or `X` character inside `[ ]`.
  * Returns the updated content, or null if the position is invalid.
  */
 export function toggleTasklistCheckbox(content: string, position: number, checked: boolean): string | null {
   const buffer = new TextEncoder().encode(content);
   // Indexes may fall off the ends and return undefined.
   if (buffer[position - 1] !== '['.charCodeAt(0) ||
-    buffer[position] !== ' '.charCodeAt(0) && buffer[position] !== 'x'.charCodeAt(0) ||
+    !' xX'.includes(String.fromCharCode(buffer[position])) ||
     buffer[position + 1] !== ']'.charCodeAt(0)) {
     return null;
   }
@@ -49,7 +49,7 @@ export function initMarkupTasklist(elMarkup: HTMLElement): void {
       if (newContent === null) {
         // Position is probably wrong. Revert and don't allow change.
         checkbox.checked = !checkbox.checked;
-        throw new Error(`Expected position to be space or x and surrounded by brackets, but it's not: position=${position}`);
+        throw new Error(`Expected position to be space, x or X and surrounded by brackets, but it's not: position=${position}`);
       }
 
       if (newContent === oldContent) {