On one's profile page, there is a Settings tab with preferences for Tag watching & ignoring, i.e. at /users/tag-notifications/<user id>.
For both Watched tags and Ignored tags there appear to be two clickable buttons that are squashed together; the "Add a tag" and "Done" buttons read as "Add a tagDone".
Both of these buttons work (clicking "Add a tag" opens a text box to add tags, and clicking "Done" closes it). But, as only one is actionable at a time, only one of these buttons ought to be displayed at a time.
Also, I don't see a good reason that the Watched tags and Ignored tags interface on the Settings page is different to that on the Questions page, i.e. at /questions.
The Questions page instead has a "Watch a tag" / "Add an ignored tag" button (or an "edit" button if populated). This brings up a text box to add/removed tags (and is closed by clicking away).
The Questions page also has the option to "Hide questions in your ignored tags" / "Gray out questions in your ignored tags", but strangely this option isn't to be found on the Settings page.
