I'm always surprised when people try to connect math and physics to CS/programming. I studied differential geometry in grad school, wrote proofs and read dense textbooks daily (very rarely missing a beat), then taught myself programming/CS. I'm now four years into my career, doing pretty well.

I see almost zero connection between the two:

* CS/programming: techniques (a mathematician might refer to these disparagingly as "tricks") used to make machines do things

* mathematics: foundational truths on the study of space, quantity, structure, change etc.

The most mathematics I've ever used on the job was basic (read: finite-dimensional) linear algebra, which is a part of every undergrad math student's basic toolkit.

If you're concerned about holes in your knowledge as a self-taught web dev, I'd study core CS courses like architecture, operating systems, networks. Know what you don't know that matters (hint: it's not "math").

EDIT: there may be a strong connection between computability theory and math https://en.wikipedia.org/wiki/Computability_theory, but I don't think this field is mainstream in either core CS (could be wrong here?) or math. I've certainly never encountered it on the job.