Joining a block from the start
In an existing contenteditable with <p>Hello World</p><p>Goodbye Space</p> in it, place the cursor before Goodbye then hit backspace to join the two paragraphs together ending with <p>Hello WorldGoodbye Space</p>

Chrome (Desktop)

Safari (Desktop)

Firefox (Desktop)

Android API 28

Note: Cursor ends in the wrong spot. It appears here WorldG|oodbye

Android API 27

Note: Cursor ends in the wrong spot. It appears here WorldG|oodbye

Android API 26

Note: Cursor ends in the wrong spot. It appears here WorldG|oodbye

Android API 25

Android API 24

Android API 23

iOS 11.4