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.

ajax_pre_render_element in core/includes/ajax.inc
Adds Ajax information about an element to communicate with JavaScript.
backdrop_common_theme in core/includes/common.inc
Provides theme registration for themes across .inc files.
FormsTestCase::testDisabledMarkup in core/modules/simpletest/tests/form.test
Verify markup for disabled form elements.
form_test_clicked_button in core/modules/simpletest/tests/form_test.module
Form builder to test button click detection.
system_element_info in core/modules/system/system.module
Implements hook_element_info().
system_jump_menu in core/modules/system/system.module
Generate a jump menu form.
_form_test_disabled_elements in core/modules/simpletest/tests/form_test.module
Build a form to test disabled elements.