How can I draw a line between two or more elements to connect them? Any combination of HTML/CSS/JavaScript/SVG/Canvas is fine.
If your answer supports any of these, then do mention it:
This question has been updated to consolidate the numerous variations of it.
jsPlumb is an option available that supports drag and drop, as seen by its numerous demos, including the Flowchart demo.
It is available in a free Community edition and a paid Toolkit edition.
The Toolkit edition wraps the Community edition with a comprehensive data binding layer, as well as several UI widgets for building applications and integrations for popular libraries, and is commercially licensed.