Open source is extremely accessible, meaning literally everyone can download it and use it.
This is a double-edged sword.
For people that want powerful sites with sophisticated applications, most of the materials are available, free. For people that want to learn how to use the software, they can use it.
However, it also means that virtually everyone can claim exposure and competence with the products.
The best qualified people will admit they have little experience, or that they are still learning. Those that are new to the field may be reluctant to admit their inexperience, for fear of missing out on an opportunity.
This is one area where requiring a certain level of related education can help differentiate between candidates that are likely to succeed and those who will struggle. The foundation knowledge provided by a computer science or software engineering degree, even if it isn’t web-specific, should help someone new to the field learn quickly and learn well.
Experience is not as reliable as a way to measure someone’s qualifications. If they have worked with something, poorly, for five years, they may not be a valuable asset.
Sample URLs can be used to assess a candidates skill level, but one must be sure their contribution is clearly stated. For example, one person might do the site design, another might do the server-side logic.
There are many self-taught web people who have excellent skills and experience, however, there are also many people who consider access and exposure to web code a license to practice - but lack the requisite skills to be part of successful projects.