What are the skills a web developer must have for a lead or senior role?

If I was hiring someone for a senior/lead web developer job I would look for a history of increasing responsibility building web sites, especially something of similar scale to what I'm working on. Domain expertise (i.e. experience in my business domain or something similar) would be a big plus. I would want the candidate to demonstrate a solid understanding of front-end, back-end, databases, security, scaling, and hosting. I would also expect a lead/senior candidate to have experience leading teams and projects, and working constructively with stakeholders.

When I have interviewed for or pitched myself (I freelance doing mostly web development and systems admin) I have focused on my experience, domain expertise, and understanding of core web technologies. I can point to multiple web sites I have written from scratch or done significant work on, and I can give references from customers, former managers, and people I have led/managed. I would expect a senior person to have similar experience, portfolio, and references.

When I was young I thought you should get promoted for being better than others at a particular thing but this is incorrect.

To lead a group in anything you need to put the team before yourself, you need to want to (and be able to) guide individuals to improve themselves, you need to have the confidence to show them the way and you need to have the courage to protect them from being hounded by others so that they can get on with their job.

IMO the most important skill is the ability to say "no" to flavour-of-the-week trends.

And before people jump up and down, that doesn't mean avoiding any new technology invented after 2005.