http://www.w3schools.com/ - surfing - this is a general reference for many web technologies. It is a great reference. it may be the overall best resource on the web for all things web development related.
http://webdesign.tutsplus.com/category/tutorials/htmlcss-tutorials/ - snorkeling - great video tutorials as well as articles
http://coding.smashingmagazine.com/ - snorkeling - a great place for more specific posts
http://alistapart.com/articles - deep diving - this is a site that goes more in depth into concepts as well as specific code examples
http://www.w3schools.com/html/ - surfing - the w3cschools html section
http://www.htmldog.com/guides/htmlbeginner/ - surfing - a great beginner resource (this site also has intermediate and advanced tutorials)
http://www.codecademy.com/tracks/htmlcss - surfing - deep diving - take these courses for as long or short as you want
http://www.abookapart.com/products/html5-for-web-designers - deep diving - read this great, yet short, book on HTML5 for a comprehensive explanation of next-gen practices
http://www.w3schools.com/css/ - surfing - a popular resource to use at any time to review a css property
http://www.htmldog.com/guides/cssbeginner/ surfing - also can find intermediate-advanced sections here
http://css-tricks.com - surfing - snorkeling - without a doubt the best site regarding CSS out there. A great resource on anything html/css/js related. Covers a lot of advanced topics
http://www.abookapart.com/products/css3-for-web-designers - deep diving - covers the next generation of css layout/appearance
http://www.w3schools.com/js/ - surfing - best way to get started
http://net.tutsplus.com/tutorials/html-css-techniques/30-html-best-practices-for-beginners/ - surfing - great beginner resource here
http://webstandardssherpa.com/ - surfing - some hits and misses here
http://www.smashingmagazine.com/web-design-essentials-examples-and-best-practices/ - surfing - an easy resource to refer to for specific elements
http://bradfrostweb.com/blog/ - snorkeling - Brad has intermediate to advanced UX topics covered
http://www.lukew.com/ff/ - deep diving - Luke is great, but a lot of stuff ventures into UX theory-ish stuff
http://www.labor.ny.gov/ux/ - surfing - deep diving - This is the NYS dept of labor UX site, this is an awesome resource
http://codepen.io/ - use this site to fiddle around with code easily and see the output without having to create html pages, etc.Very handy!.
http://foundation.zurb.com/ - Foundation 4 front-end framework
http://twitter.github.com/bootstrap/ - the bootstrap UI framework
http://stackoverflow.com/ - great forum, the most trusted on the web. use this to search for questions people have already asked, or pose your own
Don't be intimated by the vastness of the resources outlined here. We suggest for each topic you want to dive in to, take a look, see what appeals to you, and jump in. Then, over time, periodically refer to the deep diving sections to further your skills. I hope this is helpful! Enjoy!