Hi,
I don't know FormBuilder yet, but if I prepare a form like this...
$form = $modules->get("InputfieldForm");
//...
$field = $modules->get("InputfieldText");
$field->label = 'Multi<br>line<br>label';
$field->description = 'Multi<br>line<br><span style="color:red;">description</span>';
$field->attr('id+name','testfield');
$form->append($field);
//...
... to be able to pass HTML code to labels and descriptions of form fields I can set ...
$field->entityEncodeLabel = Inputfield::textFormatMarkdown;
$field->entityEncodeText = Inputfield::textFormatMarkdown;
$field->textFormat = Inputfield::textFormatMarkdown;
(ProcessWire 3.0.165, don't know whether it also applies for earlier versions...)
This can probably be used in the hook mentioned before, too.
Unfortunately a <p> tag is rendered around such a label, but I add to my CSS ...
label p { display: inline-block; margin: 0; padding: 0; }
... and then it's ok.