Gutenberg は、WordPress エディターがコンテンツとうまく連動して動くために加えられた変更です。

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Using a system of Blocks to compose and format content, the new block-based editor is designed to create rich, flexible layouts for websites and digital products. Content is created in the unit of blocks instead of freeform text with inserted media, embeds and Shortcodes (there’s a Shortcode block though).
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Blocks treat Paragraphs, Headings, Media, Embeds all as components that strung together make up the content stored in the WordPress database, replacing the traditional concept of freeform text with embeded media and shortcodes. The new editor is designed with progressive enhancement, meaning it is back-compatible with all legacy content, offers a process to try to convert and split a Classic block into block equivalents using client-side parsing and finally the blocks offer enhanced editing and format controls.
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The Editor offers rich new value to users with visual, drag-and-drop creation tools and powerful developer enhancements with modern vendor packages, reusable components, rich APIs and hooks to modify and extend the editor through Custom Blocks, Custom Block Styles and Plugins.
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