If you want to sit down and learn something new, those are one of the best places to get started.
Screencasts and interactive challenges to learn HTML and CSS. Something you should do after Codecademy, so you can repeat and enhance what you learned there.
A lot of basic and advanced tutorials. You have to search a bit to find the topics you need, but it is worth it!
Learn about Mobile First with Bootstrap, all about CSS borders, Must Know HTML5 features and many other topics.
Nobody can remember everything. Come here to look up things and find answers to everything frontend related.
If you want to look anything up regarding HTML, CSS or JS, the “Mozilla Developer Network” is by far the best resource.
Get an overview of all HTML elements and their attributes and read helpful HTML guidelines and tutorials or go through all CSS properties on that great site.
Look up CSS properties and get great tutorials about them.
This page also includes a lot of well written general blog posts about frontend development.
You have a question on how to do something or why something doesn't work as expected? There is a big chance, someone else already asked that question on Stack Overflow.
So use the search, before you add a new question there.
If you like learning while relaxing in the sun, try this book resources.
A Book Apartwww.abookapart.com
If you don't know the blog A LIST APART, you miss the best articles for ux designers, developers and content creators.
Same with their books! They are small, well written and a must read for every web professional. Their library includes: “HTML5 for Web Designers”, “CSS3 for Web Designers”, “Responsive Web Design”, “Mobile First”, “Content Strategy for Mobile”, “Designing for Emotion” and much more.
Read their articles, get the newsletter and buy their books! Enough said.