If you use code similar to the following one to define the items shown in a form, in a form builder, the rendered form contains just the markup element; the textfield will not be rendered.
$form['html'] = array(
'#type' => 'markup',
'#markup' => '<h2>My Heading</h2>',
'#tree' => true,
);
$form['html']['element'] = array(
'#type' => 'textfield',
'#title' => 'Foo',
);
The reference page for the "markup" element reports the "#tree" property as supported by the "#markup" element. The expected result would be that the child items of that element are rendered too.
Closed: won't fix
API documentation files
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
This is for the Drupal 7 Form API reference, so this is now a won't fix.
Work on documentation for supported versions of drupal is at 📌 [meta] Move/Create Form Element Documentation Active and 🌱 [Meta] Improve documentation for Render and Form Elements Active .