Short answer: Use Cmd/Ctrl+Shift+T.
Long answer: Normally (in the browser) tab focuses each element on the site that can receive input. This includes all visual elements, and includes buttons. In the Anki editor, it only focuses fields, but skips the editor buttons. The reason the tag section is skipped, is that there’s no good focus target available. Maybe what you’d expect is a new open tag to be focused, however the visual element for this new tag is only created when you click the tag icon, or click the space to the right of the last tag. Before clicking it, it doesn’t exist, so it cannot be focused.
You can also double-click images, to restore their original size (we could add this to the tooltip).