You can now preview editor content in different screen sizes. Replacing the old preview button there is now a dropdown with “Desktop”, “Tablet” and “Mobile” sizes, as well as the previous front end preview option.
The new preview options currently only work on medium to large screens (>= 600px). In the future, there may be a way to preview large screens on a phone.
If you’re a block developer and would like to have responsive styles for your blocks in the editor, use the #start-resizable-editor-section and#end-resizable-editor-section markers to define responsive styles. Styles between these markers will be picked