My definition of "passive income" is income for which you need not work more than five hours a week. Some people are stricter and define it to mean no work each week. With very few exceptions, there are no opportunities which will actually end up being passive income for you just because you're a web developer. There are several opportunities to increase your income, but it's rare that they are fully automatable (or even feasible as a part time venture).

One option is running a successful service company; this is by definition not passive unless you own it and don't need to actively manage it. So that's out.

Likewise, a productized SaaS company is not passive unless you have very minimal customer support requirements, new feature development and marketing to do. That's overwhelmingly unlikely (if you look through the routine threads asking HN for SaaS success stories, you don't typically find instances where there is a minimal time commitment).

I'm not trying to discourage you from developing a successful stream of income. I'm just trying to push back against the idea that you can reliably generate income passively using web development skills. There are a few options that can require very little time (in theory, I still don't think you'll easily get there in practice). But I think the only one that will leverage web development and still be mostly passive is running a network of websites monetized through ads that have low effort content. Other than that, maybe write a useful ebook and promote it until it has excellent SEO and referrals.