Néstor Llamas Llopis
There are about a thousand existing web develpoment courses already. Unless you can do something to stand out, don't expect too much.
What do you plan to offer that the other 1000 instructors aren't offering already?
Well, unlike all the other complete web development courses that teach "old" technologies like jQuery or CSS in .css files, here I teach all the best modern technologies and methods like React and CSS-in-JS. As I said, is a high quality and attractive course. I don't know why some people says is not recommended teaching about web development. Other says as well as a topic has high demand, doesn't matter if the current offer of courses is also high.
I'm not saying not to do it. I'm telling you that unless you can offer something the many others can't, you are going to struggle. The number of existing courses very much matters - because that means there are more people you have to differentiate yourself from. By all means, go for it. Just set some realistic expectations at the start.
As mentioned, these are very competitive topics and rest assured a LOT of FAMOUS instructors already cover all the topics you've mentioned, as part of their bundled courses. You have an uphill battle to fight to get recognized.
It isn't impossible to do, Andrei Neagoie managed to do it in 2017 and make a dent in the market. He did a very good course and incredible marketing through a medium blog post.
My recommendation, especially if you're new to online courses, would be to create a smaller niche course of 2-3 hours on a technology you know extremely well and that is under-addressed in Udemy. If that course succeeds, and you get the rating you think you want, by all means create a larger course. Your first course is rarely the best.
As far as earning potentials, make sure to check the marketplace insights tools to know how much courses are making in web dev: https://www.udemy.com/instructor/marketplace-insights/?q=Web%20Development&lang=en
Top course is 50k USD per month, and the median is $18 per month. That means if you create a course in that category, unless you're really special and have all the marketing figured out, you won't succeed in making more than $100 per month.
Best of luck!