Web development is unique compared to most other fields due to its ever evolving nature. Over the course of my career, I've collaborated with 3 types of developers; the jack, the master and the T-shaped. Shakespeare's famous quote says "a jack of all trades is a master of none, but often times better than a master of one". Here's my take on it from a web development perspective.
The jack
A jack settles for surface knowledge. They're usually quite efficient and confident in getting a bird's eye view of how connected fields work together. However, they don't specialize in any area and most tend to confidently make poor domain-specific decisions that are not well founded. As for the humble ones, they often doubt their skills and prefer to stay in the shadows.
The master
A master only settles for deep knowledge. They're experts in their field and confident in their decisions. However, they generally fail to understand how things fit together outside of their field of expertise.
The T-shaped
A T-shaped individual understands multiple areas at surface level but specializes in one. They get the best of both worlds and therefore deliver a much higher value proposition to clients and employers. One issue in being a T-shaped professional is the constant need to keep in touch, not only with what's going on in their field of expertise but also around it. It makes them more valuable while also costing them more resources (time, energy, money).
The chosen
Which one do you choose to be? Well, it comes down to your personal goals. It's a good idea to take a step back to analyze your situation and what makes the most sense to you. Personally, I chose the T-shaped way because I like to understand how things are connected to take ownership of my decisions and I also want my domain-specific skills to be valued by clients and/or employers.