. Hello, world!

add style

<style>
       #container {
           width: 1000px;
           margin: 20px auto;
       }
       .ck-editor__editable[role="textbox"] {
           /* Editing area */
           min-height: 200px;
       }
       .ck-content .image {
           /* Block images */
           max-width: 80%;
           margin: 20px auto;
       }
   </style>

now in html

<div id="container">
   <div id="editor">
   </div>
</div>

Now Script

<!-- The "super-build" of CKEditor&nbsp;5 served via CDN contains a large set of plugins and multiple editor types.See https://ckeditor.com/docs/ckeditor5/latest/installation/legacy/getting-started/quick-start.html#running-a-full-featured-editor-from-cdn-->
<script src="https://cdn.ckeditor.com/ckeditor5/41.4.2/super-build/ckeditor.js"></script>
<!--
   Uncomment to load the Spanish translation
   <script src="https://cdn.ckeditor.com/ckeditor5/41.4.2/super-build/translations/es.js"></script>
-->
<script>
   // This sample still does not showcase all CKEditor&nbsp;5 features (!)
   // Visit https://ckeditor.com/docs/ckeditor5/latest/features/index.html to browse all the features.
   CKEDITOR.ClassicEditor.create(document.getElementById("editor"), {
       // https://ckeditor.com/docs/ckeditor5/latest/getting-started/setup/toolbar/toolbar.html#extended-toolbar-configuration-format
       toolbar: {
           items: [
               'exportPDF','exportWord', '|',
               'findAndReplace', 'selectAll', '|',
               'heading', '|',
               'bold', 'italic', 'strikethrough', 'underline', 'code', 'subscript', 'superscript', 'removeFormat', '|',
               'bulletedList', 'numberedList', 'todoList', '|',
               'outdent', 'indent', '|',
               'undo', 'redo',
               '-',
               'fontSize', 'fontFamily', 'fontColor', 'fontBackgroundColor', 'highlight', '|',
               'alignment', '|',
               'link', 'uploadImage', 'blockQuote', 'insertTable', 'mediaEmbed', 'codeBlock', 'htmlEmbed', '|',
               'specialCharacters', 'horizontalLine', 'pageBreak', '|',
               'textPartLanguage', '|',
               'sourceEditing'
           ],
           shouldNotGroupWhenFull: true
       },
       // Changing the language of the interface requires loading the language file using the <script> tag.
       // language: 'es',
       list: {
           properties: {
               styles: true,
               startIndex: true,
               reversed: true
           }
       },
       // https://ckeditor.com/docs/ckeditor5/latest/features/headings.html#configuration
       heading: {
           options: [
               { model: 'paragraph', title: 'Paragraph', class: 'ck-heading_paragraph' },
               { model: 'heading1', view: 'h1', title: 'Heading 1', class: 'ck-heading_heading1' },
               { model: 'heading2', view: 'h2', title: 'Heading 2', class: 'ck-heading_heading2' },
               { model: 'heading3', view: 'h3', title: 'Heading 3', class: 'ck-heading_heading3' },
               { model: 'heading4', view: 'h4', title: 'Heading 4', class: 'ck-heading_heading4' },
               { model: 'heading5', view: 'h5', title: 'Heading 5', class: 'ck-heading_heading5' },
               { model: 'heading6', view: 'h6', title: 'Heading 6', class: 'ck-heading_heading6' }
           ]
       },
       // https://ckeditor.com/docs/ckeditor5/latest/features/editor-placeholder.html#using-the-editor-configuration
       placeholder: 'Welcome to CKEditor 5!',
       // https://ckeditor.com/docs/ckeditor5/latest/features/font.html#configuring-the-font-family-feature
       fontFamily: {
           options: [
               'default',
               'Arial, Helvetica, sans-serif',
               'Courier New, Courier, monospace',
               'Georgia, serif',
               'Lucida Sans Unicode, Lucida Grande, sans-serif',
               'Tahoma, Geneva, sans-serif',
               'Times New Roman, Times, serif',
               'Trebuchet MS, Helvetica, sans-serif',
               'Verdana, Geneva, sans-serif'
           ],
           supportAllValues: true
       },
       // https://ckeditor.com/docs/ckeditor5/latest/features/font.html#configuring-the-font-size-feature
       fontSize: {
           options: [ 10, 12, 14, 'default', 18, 20, 22 ],
           supportAllValues: true
       },
       // Be careful with the setting below. It instructs CKEditor to accept ALL HTML markup.
       // https://ckeditor.com/docs/ckeditor5/latest/features/general-html-support.html#enabling-all-html-features
       htmlSupport: {
           allow: [
               {
                   name: /.*/,
                   attributes: true,
                   classes: true,
                   styles: true
               }
           ]
       },
       // Be careful with enabling previews
       // https://ckeditor.com/docs/ckeditor5/latest/features/html-embed.html#content-previews
       htmlEmbed: {
           showPreviews: false
       },
       // https://ckeditor.com/docs/ckeditor5/latest/features/link.html#custom-link-attributes-decorators
       link: {
           decorators: {
               addTargetToExternalLinks: true,
               defaultProtocol: 'https://',
               toggleDownloadable: {
                   mode: 'manual',
                   label: 'Downloadable',
                   attributes: {
                       download: 'file'
                   }
               }
           }
       },
       // https://ckeditor.com/docs/ckeditor5/latest/features/mentions.html#configuration
       mention: {
           feeds: [
               {
                   marker: '@',
                   feed: [
                       '@apple', '@bears', '@brownie', '@cake', '@cake', '@candy', '@canes', '@chocolate', '@cookie', '@cotton', '@cream',
                       '@cupcake', '@danish', '@donut', '@dragée', '@fruitcake', '@gingerbread', '@gummi', '@ice', '@jelly-o',
                       '@liquorice', '@macaroon', '@marzipan', '@oat', '@pie', '@plum', '@pudding', '@sesame', '@snaps', '@soufflé',
                       '@sugar', '@sweet', '@topping', '@wafer'
                   ],
                   minimumCharacters: 1
               }
           ]
       },
       // The "superbuild" contains more premium features that require additional configuration, disable them below.
       // Do not turn them on unless you read the documentation and know how to configure them and setup the editor.
       removePlugins: [
           // These two are commercial, but you can try them out without registering to a trial.
           // 'ExportPdf',
           // 'ExportWord',
           'AIAssistant',
           'CKBox',
           'CKFinder',
           'EasyImage',
           // This sample uses the Base64UploadAdapter to handle image uploads as it requires no configuration.
           // https://ckeditor.com/docs/ckeditor5/latest/features/images/image-upload/base64-upload-adapter.html
           // Storing images as Base64 is usually a very bad idea.
           // Replace it on production website with other solutions:
           // https://ckeditor.com/docs/ckeditor5/latest/features/images/image-upload/image-upload.html
           // 'Base64UploadAdapter',
           'MultiLevelList',
           'RealTimeCollaborativeComments',
           'RealTimeCollaborativeTrackChanges',
           'RealTimeCollaborativeRevisionHistory',
           'PresenceList',
           'Comments',
           'TrackChanges',
           'TrackChangesData',
           'RevisionHistory',
           'Pagination',
           'WProofreader',
           // Careful, with the Mathtype plugin CKEditor will not load when loading this sample
           // from a local file system (file://) - load this site via HTTP server if you enable MathType.
           'MathType',
           // The following features are part of the Productivity Pack and require additional license.
           'SlashCommand',
           'Template',
           'DocumentOutline',
           'FormatPainter',
           'TableOfContents',
           'PasteFromOfficeEnhanced',
           'CaseChange'
       ]
   });
</script>