Note: this list is generated by looking for the string for this theme hook, so it may include some references that are not actually using this theme hook.

BackdropWebTestCase::handleForm in core/modules/simpletest/backdrop_web_test_case.php
Handle form input related to backdropPost(). Ensure that the specified fields exist and attempt to create POST data in the correct manner for the particular field type.
backdrop_common_theme in core/includes/
Provides theme registration for themes across .inc files.
backdrop_strip_dangerous_protocols in core/includes/
Strips dangerous protocols (e.g. 'javascript:') from a URI.
contact_personal_form in core/modules/contact/
Form constructor for the personal contact form.
contact_site_form in core/modules/contact/
Form constructor for the site-wide contact form.
FormElementTestCase::testPlaceHolderText in core/modules/simpletest/tests/form.test
Tests placeholder text for elements that support placeholders.
FormsTestCase::testRequiredFields in core/modules/simpletest/tests/form.test
Check several empty values for required forms elements.
form_test_placeholder_test in core/modules/simpletest/tests/form_test.module
Builds a form to test the placeholder attribute.
system_element_info in core/modules/system/system.module
Implements hook_element_info().
x21.php in core/includes/transliteration/x21.php
xd1.php in core/includes/transliteration/xd1.php
_filter_url in core/modules/filter/filter.module
Implements callback_filter_process().
_form_test_disabled_elements in core/modules/simpletest/tests/form_test.module
Build a form to test disabled elements.