Content placeholder

import ClassicEditor from '@ckeditor/ckeditor5-editor-classic/src/classiceditor';
import Essentials from '@ckeditor/ckeditor5-essentials/src/essentials';
import Paragraph from '@ckeditor/ckeditor5-paragraph/src/paragraph';
import Heading from '@ckeditor/ckeditor5-heading/src/heading';
import List from '@ckeditor/ckeditor5-list/src/list';
import Bold from '@ckeditor/ckeditor5-basic-styles/src/bold';
import Italic from '@ckeditor/ckeditor5-basic-styles/src/italic';

import Plugin from '@ckeditor/ckeditor5-core/src/plugin';
import { toWidget, viewToModelPositionOutsideModelElement } from '@ckeditor/ckeditor5-widget/src/utils';
import Widget from '@ckeditor/ckeditor5-widget/src/widget';
import Command from '@ckeditor/ckeditor5-core/src/command';

import { addListToDropdown, createDropdown } from '@ckeditor/ckeditor5-ui/src/dropdown/utils';
import Collection from '@ckeditor/ckeditor5-utils/src/collection';
import Model from '@ckeditor/ckeditor5-ui/src/model';

class PlaceholderCommand extends Command {
    execute( { value } ) {
        const editor = this.editor;
        const selection = editor.model.document.selection;

        editor.model.change( writer => {
            // Create a <placeholder> element with the "name" attribute (and all the selection attributes)...
            const placeholder = writer.createElement( 'placeholder', {
                ...Object.fromEntries( selection.getAttributes() ),
                name: value
            } );

            // ... and insert it into the document.
            editor.model.insertContent( placeholder );

            // Put the selection on the inserted element.
            writer.setSelection( placeholder, 'on' );
        } );
    }

    refresh() {
        const model = this.editor.model;
        const selection = model.document.selection;

        const isAllowed = model.schema.checkChild( selection.focus.parent, 'placeholder' );

        this.isEnabled = isAllowed;
    }
}

class Placeholder extends Plugin {
    static get requires() {
        return [ PlaceholderEditing, PlaceholderUI ];
    }
}

class PlaceholderUI extends Plugin {
    init() {
        const editor = this.editor;
        const t = editor.t;
        const placeholderNames = editor.config.get( 'placeholderConfig.types' );

        // The "placeholder" dropdown must be registered among the UI components of the editor
        // to be displayed in the toolbar.
        editor.ui.componentFactory.add( 'placeholder', locale => {
            const dropdownView = createDropdown( locale );

            // Populate the list in the dropdown with items.
            addListToDropdown( dropdownView, getDropdownItemsDefinitions( placeholderNames ) );

            dropdownView.buttonView.set( {
                // The t() function helps localize the editor. All strings enclosed in t() can be
                // translated and change when the language of the editor changes.
                label: t( 'Placeholder' ),
                tooltip: true,
                withText: true
            } );

            // Disable the placeholder button when the command is disabled.
            const command = editor.commands.get( 'placeholder' );
            dropdownView.bind( 'isEnabled' ).to( command );

            // Execute the command when the dropdown item is clicked (executed).
            this.listenTo( dropdownView, 'execute', evt => {
                editor.execute( 'placeholder', { value: evt.source.commandParam } );
                editor.editing.view.focus();
            } );

            return dropdownView;
        } );
    }
}

function getDropdownItemsDefinitions( placeholderNames ) {
    const itemDefinitions = new Collection();

    for ( const name of placeholderNames ) {
        const definition = {
            type: 'button',
            model: new Model( {
                commandParam: name,
                label: name,
                withText: true
            } )
        };

        // Add the item definition to the collection.
        itemDefinitions.add( definition );
    }

    return itemDefinitions;
}

class PlaceholderEditing extends Plugin {
    static get requires() {
        return [ Widget ];
    }

    init() {
        console.log( 'PlaceholderEditing#init() got called' );

        this._defineSchema();
        this._defineConverters();

        this.editor.commands.add( 'placeholder', new PlaceholderCommand( this.editor ) );

        this.editor.editing.mapper.on(
            'viewToModelPosition',
            viewToModelPositionOutsideModelElement( this.editor.model, viewElement => viewElement.hasClass( 'placeholder' ) )
        );
        this.editor.config.define( 'placeholderConfig', {
            types: [ 'date', 'first name', 'surname' ]
        } );
    }

    _defineSchema() {
        const schema = this.editor.model.schema;

        schema.register( 'placeholder', {
            // Allow wherever text is allowed:
            allowWhere: '$text',

            // The placeholder will act as an inline node:
            isInline: true,

            // The inline widget is self-contained so it cannot be split by the caret and it can be selected:
            isObject: true,

            // The inline widget can have the same attributes as text (for example linkHref, bold).
            allowAttributesOf: '$text',

            // The placeholder can have many types, like date, name, surname, etc:
            allowAttributes: [ 'name' ]
        } );
    }

    _defineConverters() {
        const conversion = this.editor.conversion;

        conversion.for( 'upcast' ).elementToElement( {
            view: {
                name: 'span',
                classes: [ 'placeholder' ]
            },
            model: ( viewElement, { writer: modelWriter } ) => {
                // Extract the "name" from "{name}".
                const name = viewElement.getChild( 0 ).data.slice( 1, -1 );

                return modelWriter.createElement( 'placeholder', { name } );
            }
        } );

        conversion.for( 'editingDowncast' ).elementToElement( {
            model: 'placeholder',
            view: ( modelItem, { writer: viewWriter } ) => {
                const widgetElement = createPlaceholderView( modelItem, viewWriter );

                // Enable widget handling on a placeholder element inside the editing view.
                return toWidget( widgetElement, viewWriter );
            }
        } );

        conversion.for( 'dataDowncast' ).elementToElement( {
            model: 'placeholder',
            view: ( modelItem, { writer: viewWriter } ) => createPlaceholderView( modelItem, viewWriter )
        } );

        // Helper method for both downcast converters.
        function createPlaceholderView( modelItem, viewWriter ) {
            const name = modelItem.getAttribute( 'name' );

            const placeholderView = viewWriter.createContainerElement( 'span', {
                class: 'placeholder'
            } );

            // Insert the placeholder name (as a text).
            const innerText = viewWriter.createText( '{' + name + '}' );
            viewWriter.insert( viewWriter.createPositionAt( placeholderView, 0 ), innerText );

            return placeholderView;
        }
    }
}

ClassicEditor
    .create( document.querySelector( '#snippet-inline-widget' ), {
        plugins: [ Essentials, Paragraph, Heading, List, Bold, Italic, Placeholder ],
        toolbar: {
            items: [
                'undo', 'redo',
                '|', 'placeholder',
                '|', 'heading',
                '|', 'bold', 'italic',
                '|', 'link', 'uploadImage', 'insertTable', 'mediaEmbed',
                '|', 'bulletedList', 'numberedList', 'outdent', 'indent'
            ]
        },
        placeholderConfig: {
            types: [ 'date', 'color', 'first name', 'surname' ]
        },
        ui: {
            viewportOffset: {
                top: window.getViewportTopOffsetConfig()
            }
        }
    } )
    .then( editor => {
        console.log( 'Editor was initialized', editor );

        // Expose for playing in the console.
        window.editor = editor;
    } )
    .catch( error => {
        console.error( error.stack );
    } );

<style>
    .placeholder {
        background: #ffff00;
        padding: 4px 2px;
        outline-offset: -2px;
        line-height: 1em;
        margin: 0 1px;
    }

    .placeholder::selection {
        display: none;
    }
</style>

<div id="snippet-inline-widget">
    <p>Hello <span class="placeholder">{first name}</span> <span class="placeholder">{surname}</span>!</p>
</div>