Source
File: wp-includes/widgets/class-wp-widget-custom-html.php
public static function render_control_template_scripts() {
?>
<script type="text/html" id="tmpl-widget-custom-html-control-fields">
<# var elementIdPrefix = 'el' + String( Math.random() ).replace( /\D/g, '' ) + '_' #>
<p>
<label for="{{ elementIdPrefix }}title"><?php esc_html_e( 'Title:' ); ?></label>
<input id="{{ elementIdPrefix }}title" type="text" class="widefat title">
</p>
<p>
<label for="{{ elementIdPrefix }}content" id="{{ elementIdPrefix }}content-label"><?php esc_html_e( 'Content:' ); ?></label>
<textarea id="{{ elementIdPrefix }}content" class="widefat code content" rows="16" cols="20"></textarea>
</p>
<?php if ( ! current_user_can( 'unfiltered_html' ) ) : ?>
<?php
$probably_unsafe_html = array( 'script', 'iframe', 'form', 'input', 'style' );
$allowed_html = wp_kses_allowed_html( 'post' );
$disallowed_html = array_diff( $probably_unsafe_html, array_keys( $allowed_html ) );
?>
<?php if ( ! empty( $disallowed_html ) ) : ?>
<# if ( data.codeEditorDisabled ) { #>
<p>
<?php _e( 'Some HTML tags are not permitted, including:' ); ?>
<code><?php echo join( '</code>, <code>', $disallowed_html ); ?></code>
</p>
<# } #>
<?php endif; ?>
<?php endif; ?>
<div class="code-editor-error-container"></div>
</script>
<?php
}