I've never heard of a master's degree in web development. Sounds kinda hokey. Anyway, I was in a similar situation with 2 bachelor's degrees completely unrelated to the computer industry. Due to life circumstances I wasn't able to pursue a career in any of those fields but needed work pretty quick and always had an interest in technology. So I got a job as a sys admin.

After a few years into that I began to teach myself programming. My local university at the time started a Master's program in Computer Science a few years later. I applied for several reasons that may help with your thought process:

• My job offered an automatic raise to anybody who had a Master's degree. This was probably the biggest motivation for me.
• While I learned a great deal about programming on my own, I lacked a comprehensive background in certain topics such as algorithms and software engineering, and felt like I needed a formal education in those areas to maximize my potential.
• I was around a lot of people who had CS degrees and, frankly, felt a little insecure that I didn't. Probably a little stupid, but that's how I felt at the time.
• I loved college, and going back to study for my Master's was not something I dreaded.
• I was able to go without taking on a lot of debt.

All this being said, you can make a go of it either way you choose. As others have said, you can learn everything you need to know on your own. The reasons I've outlined were just the reasons for my choice which may or may not apply to you. As for the online certifications, I wouldn't put too much time in this route even if you choose not to pursue your Master's. Just focus on learning and building a solid portfolio.