TYPO3 Core

can reproduce (master).

Minimal test case that renders the erroneous

required=required

type: Form
identifier: testForm
prototypeName: standard
renderables:
  -
    type: Page
    identifier: page-1
    renderables:
      -
        type: Text
        identifier: text-1
        properties:
          fluidAdditionalAttributes:
            required: required
      -
        type: Text
        identifier: text-2

Mind that it can change behaviour on the 2nd reload after a cache clear and that it is not consistent!

... we just found out that wrapping the fields inside GridRow->Fieldset->... greatly reduces the probability of seeing the error... It is getting weird...

As such you can also reproduce it with the introduction distribution aka bootstrap_package with typo3conf/ext/bootstrap_package/Resources/Private/Forms/Contact.form.yaml on current master, although I had to cache clear+reload ~10 times. Btw: Testing is easier if you remove the template typo3conf/ext/bootstrap_package/Resources/Private/Templates/Form/Form.html (or at least the line novalidate=) as then you do not have to use browser dev tools to see the error but you get erroneous HTML5-validation straight away.

Fun fact: typo3conf/ext/bootstrap_package/Resources/Private/Templates/Form/Form.html does only add the line novalidate= to the default Form.html template.

This actually is a workaround for this bug. You get wrong HTML then, but it does not interfere with your HTML5-validation.

I can reproduce this with the testcase from #86890#note-3 with TYPO3 9.5.2-dev. TYPO3 8.7-x is not affected.
The issue occurs after clearing the system cache and after a reload of the form page.
After the fluid templates have been cached, the error disappears.
I'll try to find a solution.

It is reproducible with plain fluid:

This works as expected:

<f:form>
    <f:form.textfield additionalAttributes="{required: 'required'}" />
    <f:form.textfield additionalAttributes="{}" />
</f:form>

Result:

<form action="/" method="post">
    <input required="required" type="text" name="">
    <input type="text" name="">
</form>

but with a foreach:

<f:form>
    <f:for each="{0: {required: 'required'}, 1: '{}'}" as="attribs">
        <f:form.textfield additionalAttributes="{attribs}" />
    </f:for>
</f:form>

Result:

<form action="/" method="post">
    <input required="required" type="text" name="">
    <input required="required" type="text" name="">
</form>

You can disable the fluid template cache to reproduce the issue all the time:

$GLOBALS['TYPO3_CONF_VARS']['SYS']['caching']['cacheConfigurations']['fluid_template']['backend'] = \TYPO3\CMS\Core\Cache\Backend\NullBackend::class;

I continue to debug this.

The TYPO3 CMS AbstractTagBasedViewHelper implementation (https://github.com/TYPO3/TYPO3.CMS/blob/TYPO3_8-7/typo3/sysext/fluid/Classes/Core/ViewHelper/AbstractTagBasedViewHelper.php) is removed since (https://github.com/TYPO3/TYPO3.CMS/commit/533a1a349e99570a9e59357f43277add5a74b636#diff-37c6a16d3e2de976d66e17b797cda2ba).
The initialize() method differs a little bit.

https://github.com/TYPO3/TYPO3.CMS/blob/TYPO3_8-7/typo3/sysext/fluid/Classes/Core/ViewHelper/AbstractTagBasedViewHelper.php#L83 vs https://github.com/TYPO3/Fluid/blob/2.5.4/src/Core/ViewHelper/AbstractTagBasedViewHelper.php#L88

Within typo3/sysext/fluid/Classes/Core/ViewHelper/AbstractTagBasedViewHelper.php (TYPO3 8.7) there was this code

$this->tag->reset();
$this->tag->setTagName($this->tagName);

which reset the tagbuilder.
Now this reset will never happened at least with the usage of the Textfield ViewHelper.

Amazing find, Ralf!

For the current fluid in master (2.5.2) this fixes it for me:

--- fluid_orig/src/Core/ViewHelper/AbstractTagBasedViewHelper.php       2018-11-16 10:13:58.885022862 +0100
+++ fluid/src/Core/ViewHelper/AbstractTagBasedViewHelper.php    2018-11-16 10:14:14.961016742 +0100
@@ -88,6 +88,8 @@
     public function initialize()
     {
         parent::initialize();
+        $this->tag->reset();
+        $this->tag->setTagName($this->tagName);

         if ($this->hasArgument('additionalAttributes') && is_array($this->arguments['additionalAttributes'])) {
             $this->tag->addAttributes($this->arguments['additionalAttributes']);

Pull request is upstream: https://github.com/TYPO3/Fluid/pull/419

Fluid 2.6 is released which fixes the bug.

But it is not included in TYPO3 master yet.

Patch set 5 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/59554

Patch set 6 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/59554

Patch set 1 for branch 9.5 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/59609