A proper webdev project manager understands how to build what needs to be built. This person may not possess the skills to do it themselves, but they get what it takes to make the project happen.
Typically, companies bring in project managers from another discipline, or hire people with no experience. Neither of these strategies work. That’s because every industry has its own idiosyncrasies.
As much as any other team member, the project manager needs to know what those are for each type of project. This includes being familiar with industry jargon. For instance, the webdev project manager needs to be able to recognize if a screen design isn’t responsive, or what a wireframe is, etc.
Project managers have to be able to be pushy when needed. They are the energy source that keeps the project moving. They must be willing and able to constantly and proactively follow up on everything that needs to be happening at any given time. This may result in them being considered a pain in the backside, but it's their job to do what must be done to get the job over the finish line on time.
Think of it as having a velvet glove with an iron fist inside: Social lubrication must be kept intact, but people have to be held accountable for deliverables.