Beginning with WordPress 5, Gutenberg will be part of the WordPress core. If you want a preview, install it as a plugin. What is Gutenberg?
The Gutenberg WordPress editor is a new page builder that is being designed to integrate with WordPress core. Gutenberg will add content blocks and page builder-like functionality to every up-to-date WordPress website. When in use, it will replace TinyMCE as the default content editor. With Gutenberg, content is added in blocks of various types from the WordPress backend. This is a quote block.From: iThemes
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