You can probably figure this out for yourself. Here's my non-patented general approach to figuring out the current job market for a given skill set.
Go to a few of the major job sites such as Monster. Run a few searches for terms like "web developer", and the skills you're interested in e.g. PHP, HTML5, JavaScript, CSS etc. Make a note of the number of jobs available and the pay rates for various skills and whether you see any sign of career development e.g. if all the jobs are for "junior PHP developer", you might wonder what you're supposed to do after a couple of years of "junior PHP development". For example, here in the UK there seem to be lots of jobs for PHP programmers, but they're mostly on fairly low pay as far as I can tell.
If there are lots of jobs and high pay, then it's probably an area where there is a genuine shortage of skills. But other people will also be noticing this. If there are lots of jobs on low pay, then it's probably an area with high turnover or a lot of short-term work, but no particular skills shortage and/or a lot of people already doing the work very cheaply. If there are not many jobs but high pay, then this might be a new area where cutting-edge skills are hard to find (unlikely in this case), or it might be an old area where legacy skills are hard to find. If there are not many jobs and they're all on low pay, then you probably don't want to get into it anyway.
And keep an eye on industry news via blogs, websites, corporate news, tech journals etc to get a feel for the general direction things are heading in your area of interest.
But
you should think about what you mean by a "web developer". These days, a huge proportion of applications are browser-based i.e. they are web applications, so web development skills are certainly needed. But you might not get very far in e.g. a classic enterprise
Java shop if you can only offer HTML/CSS. And with powerful JavaScript functionality in the browser being complemented by JavaScript moving into the server-side, assorted options for generating client-side JS from Java/Clojure etc, and the massive growth in mobile applications using different technologies, the boundaries between "web development" and many other areas of application development are getting pretty blurry.
Even if you can find web-only niches (not sure what these would look like - presumably front-end browser stuff etc), there will inevitably be more jobs for software developers who can offer other skills as well as "web development". You might want to aim for being a good software developer, specialising in web development because that's where your current interests lie, but knowing at least something about the other tools/technologies that will be relevant to many web applications. In any case, if you want a career as any kind of developer (or to progress in that career), then you need to be prepared to refresh and extend your skill-set regularly. You can't expect to stay your entire career doing the same thing in this industry.
So figure out what you want from a job - high pay, cool technology, interesting work, convenient location, future opportunities, security etc, then adjust your job search criteria accordingly.
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