editor-style.php

<?php

/* STYLES IN EDITOR - Multiple stylesheets
   ================================================== */
function child_load_editor_mce_css( $mce_css ) {
	if ( ! empty( $mce_css ) ) {
	    $mce_css .= ',';
	}
	
  $mce_css .= BW_CHILD_URL_DIST . '/bootstrap/css/bootstrap.css';
  $mce_css .= ',';
  $mce_css .= BW_CHILD_URL_DIST . '/assets/css/editor.css';
  
	return $mce_css;
}
add_filter( 'mce_css', 'child_load_editor_mce_css' );




/* STYLES IN EDITOR - Single Stylesheet
   ================================================== */
function twbsg_mce_css( $mce_css ) {
	if ( ! empty( $mce_css ) ) {
		$mce_css .= ',';
	}
	$mce_css .= plugins_url('bootstrap/css/bootstrap.min.css', __DIR__); // path to file
	return $mce_css;
}
add_filter( 'mce_css', 'twbsg_mce_css' );

styleselect-html5.php

<?php

function mce_html5_formatting( $init ) {
    $init['block_formats'] = 'Paragraph=p;Heading 1=h1;Heading 2=h2;Heading 3=h3;Heading 4=h4;Heading 5=h5;Heading 6=h6;';

    $style_formats = array(
        array(
            'title' => 'Headers',
                'items' => array(
                array(
                    'title' => 'Header 1',
                    'block' => 'h1'
                ),
                array(
                    'title' => 'Header 2',
                    'block' => 'h2'
                ),
		array(
                    'title' => 'Header 3',
                    'block' => 'h3'
                ),
		array(
                    'title' => 'Header 4',
                    'block' => 'h4'
                ),
		array(
                    'title' => 'Header 5',
                    'block' => 'h5'
                ),
		array(
                    'title' => 'Header 6',
                    'block' => 'h6'
                )
            )
        ),
        array(
            'title' => 'Blocks',
                'items' => array(
                array(
                    'title' => 'Paragraph <p>',
                    'block' => 'p'
                ),
                array(
                    'title' => 'Arbitrary Division <div>',
                    'block' => 'div'
                ),
		array(
                    'title' => 'Preformatted Text <pre>',
                    'block' => 'pre'
                )
            )
        ),
        array(
            'title' => 'Containers',
                'items' => array(
                array(
                    'title' => 'Section',
                    'block' => 'section',
		    'wrapper' => 'true',
                     'merge_siblings' => 'false'
                ),
                array(
                    'title' => 'Blockquote',
                    'block' => 'blockquote',
		    'wrapper' => 'true',
		    'icon' => 'blockquote'
                ),
                array(
                    'title' => 'Figure',
                    'block' => 'figure',
		    'wrapper' => 'true'
                )
            )
        )
    );

    $init['style_formats'] = json_encode( $style_formats );

    $init['style_formats_merge'] = false;
    
    $init['content_style'] = "#tinymce {margin: 10px 15px!important;}";

    return $init;
}
add_filter('tiny_mce_before_init', 'mce_html5_formatting');



function mce_add_buttons_styleselect( $buttons ){
    array_splice( $buttons, 1, 0, 'styleselect' );
    return $buttons;
}
add_filter( 'mce_buttons_2', 'mce_add_buttons_styleselect' );