This is from the perspective of someone who works as part of a team of developers, who are one of several teams working on different projects for the same company.
1. Can you tell me about the day-to-day responsibilities of a web developer?
Coordinating with team members to determine who needs to work on what in order to best meet short-term goals, then doing what you committed to doing.
2. What do you think are the most important qualities for someone to excel in this field?
Love to solve puzzles and hate giving up on difficult ones. Always ready to learn new things.
3. What education or certifications do you recommend for being competitive as a developer?
With the cost of college these days, at least in most of the USA, I'm not sure it's worth a 4-year comp. sci. degree if it's going to put you into debt -- at least not for a lot of web dev work. (Programmers for NASA, that's a different story.) Some of the intense "boot camps" that are available to learn specific skills in a few months may get you into an intro-level job 2 or 3 years sooner than a 4-year bachelors degree, so there are some difficult decisions to make these days in this regard.
4. Is web development a primary responsibility or is it more commonly a ‘side job’ for other types of jobs i.e. graphic design?
I my case, most of the developers where I work are involved in some aspect of web development, as that is essentially what our products are. A minority are instead data analysis/processing devs, along with a few dev-ops specialists.
5. What are the biggest challenges a web developer faces.
Constantly changing browsers ( ), tight schedules, poorly thought out business requirements, new and changing languages and tools to keep up with.
6. How difficult is it to find a job as a web developer?
Good programmers with good people skills and positive attitudes should have very little problem finding jobs. Mediocre programmers who do not "play nicely with others" may find it difficult to compete.
7. Is work done with firms, or is it more often freelance work?
I tried being a freelancer for a couple years and hated it. Now I've been part of a dot.com-type company for over 5 years and am much happier in that sort of environment, working with a great team of people I really like.
8. What professional associations are prevalent in the field? Do you belong to any of them?
I don't really know, and no.
9. What does an effective portfolio look like as a web designer?
I am not a web designer, so don't know. (I'm mainly a server-side specialist with fairly rudimentary client-side skills and very limited artistic abilities.)
10. If you could go back to before your started this career path and give yourself some advice, what would it be?
Start earlier. (It was a long, curious path that led to me getting my first full-time software developer job in my mid-50's. )