RPi Projects has tutorials to teach the languages of the web with funny projects like a meme generator, pixel art, an internet cookbook, and a zombie survival map!
Other Intro Tutorials
Khan Academy (HTML/CSS)
Mozilla - the people who make Firefox - have created six fun activities for learning to make web pages. Check it out!
Hacking the code of an existing website can be a fun way to dig into web design!
If you want to start from a blank slate and build your own web page, codepen is a great tool. Need hints or refreshers? Check out W3Schools.