Front-end developers should learn and master, obviously, still the same - front-end development.
Read more in my answer to What languages, frameworks and libraries should I learn in order to start front-end development?.
Another thing engineer should focus on - becoming a senior and efficient software engineer.
Last additional note about UI engineers - master design thinking, business analysis, UI/UX design, UX architecture and engineering, designer-engineer cooperation and tools.
Everything else is secondary and depends on project.