As much as we try--CSS, GUI, and all things Front-end including graphics have long become their own science, even for Web Developers.

Is is true? Should programmers stick to what they know best (which, in most cases, is Programming)?

Trying to know as much as you can wouldn't hurt, but in a professional working environment, different people handle different areas, analysis, design, coding, debugging and stuff like that.
Take for instance a game company (cd projekt red.
They need the following people:
>>>Animator >>>Composer
>>>Art Tools Programmer >>>Concept Artist
>>>Audio Director >>>Cultural Consultant
>>>Audio Programmer >>>Design Director
>>>Audio Tools Programmer >>>Environment Artist
>>>Build Engineer >>>Gameplay Designer
>>>Business Development Manager
>>>Gameplay Programmer
>>>Character Artist >>>GFX/Shader Programmer
>>>Cinematic Designer >>>Graphics Programmer
>>>Comic Illustrator >>>Illustrator
>>>Junior Engine Programmer>>>Lead Gameplay Designer
>>>Lead Producer >>>Lead Vehicle Artist
>>>Level Designer >>>Lighting Artist
>>>Localization Programmer >>>Mocap Specialist
>>>Senior Weapons Artist >>>Motion Designer
>>>Network Programmer >>>Open World Designer
>>>Pipeline Programmer >>>Producer
>>>Product Manager >>>Project Manager
>>>Quest Designer >>>Physics Programmer
>>>Senior Software Engineer – SDK (C++)
>>> Senior Unity Developer >>>Sound Designer
>>>Technical Artist >>>UI/UX Artist
>>>UI Designer >>>UI Programmer
>>>Vfx Artist
Do i need to say more, why would i be good at programming still be trying to learn graphics and using engines. As seen above Ui artist/designer/programmer are even all different. Not to talk of Gameplay Designer/programmer. Design is highly
Differentiated from programming. That's why i laugh when i see people who are good in programming struggling with things like graphics and ui. Well knowledge isn't bad, but when working in a large environment, there would be specialization

zzzzy:
Trying to know as much as you can wouldn't hurt, but in a professional working environment, different people handle different areas, analysis, design, coding, debugging and stuff like that.
Take for instance a game company (cd projekt red.
They need the following people:
>>>Animator >>>Composer
>>>Art Tools Programmer >>>Concept Artist
>>>Audio Director >>>Cultural Consultant
>>>Audio Programmer >>>Design Director
>>>Audio Tools Programmer >>>Environment Artist
>>>Build Engineer >>>Gameplay Designer
>>>Business Development Manager
>>>Gameplay Programmer
>>>Character Artist >>>GFX/Shader Programmer
>>>Cinematic Designer >>>Graphics Programmer
>>>Comic Illustrator >>>Illustrator
>>>Junior Engine Programmer>>>Lead Gameplay Designer
>>>Lead Producer >>>Lead Vehicle Artist
>>>Level Designer >>>Lighting Artist
>>>Localization Programmer >>>Mocap Specialist
>>>Senior Weapons Artist >>>Motion Designer
>>>Network Programmer >>>Open World Designer
>>>Pipeline Programmer >>>Producer
>>>Product Manager >>>Project Manager
>>>Quest Designer >>>Physics Programmer
>>>Senior Software Engineer – SDK (C++)
>>> Senior Unity Developer >>>Sound Designer
>>>Technical Artist >>>UI/UX Artist
>>>UI Designer >>>UI Programmer
>>>Vfx Artist
Do i need to say more, why would i be good at programming still be trying to learn graphics and using engines. As seen above Ui artist/designer/programmer are even all different. Not to talk of Gameplay Designer/programmer. Design is highly
Differentiated from programming. That's why i laugh when i see people who are good in programming struggling with things like graphics and ui. Well knowledge isn't bad, but when working in a large environment, there would be specialization

Nice.

Specialization rocks . . . . If your present enterprise swells to a monolithic scale.

UI/UX design is a whole 'nother science if you ask me and should not be hastily thought out and implemented to beat deadlines and hit milestones.

I still plan out the UI/UX of my projects but I also hire a dedicated UX specialist to rethink the Information architecture and logical flow of my applications.

Last word: Learn UI/UX . . . just for the kicks and if you've got real affinity for it . . . but stick to coding.