If it were me, I would talk about the history of Web Development to explain with how we arrived at the current state, and maybe offer a couple of futuristic teasers of where it could go. People are much more likely to respond to a story, than a 'hey here's how it works' but that largely depends on the audience.
Something that would look like: - Netscape Browser and the Rise of HTML - Introduction of CSS and JavaScript (explain how separation of Content/Function/CSS is a great thing) - Talk about the rise of Ruby/Rails/Jquery CMS systems and how specialized tools developed for different areas of the stack. - Talk about the rise of SPA architecture and how many things are being abstracted away. - Talk about future technologies and possible future trends in Web Development