Focus should be in the editor when the page is loaded.

[lrhn]

When I go to dartpad.dev (writing dartpad.dev in the address line and pressing return), the page loads, but focus is no-where.

The focus should be in the source editor component, where you are probably planning to write something. (I have to move my hand to the mouse to get focus on the editor. Using Tab does nothing, so not only is focus not in the editor, it's nowhere, and normal tab navigation is broken for the page.)