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JSBin (jsbin.com) -- really helpful tool for messing around in HTML/CSS/JS. Make sure to log in with your GitHub account (and make a GitHub if you don't already have one) so that all your work is saved
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The Code Player (http://thecodeplayer.com/) -- nifty site with "screencast"-style animations to watch how web apps and effects are built
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Mozilla Developer Network (MDN) documentation (https://developer.mozilla.org/en-US/docs/Web) -- if you need to reference anything related to web technology, like looking up JavaScript functions or CSS properties, use this website. You should never ever ever ever use W3Schools.org -- they are bad and wrong and always use MDN instead
- Responsive web design guide (http://blog.teamtreehouse.com/beginners-guide-to-responsive-web-design) -- a good introduction to responsive design principles like relative size units (rems, ems, percentages) and @media queries
- MDN HTML documentation (https://developer.mozilla.org/en-US/docs/Web/HTML/Element) has documentation on all the semantic elements
- CSS selectors: levels 1-10 (http://flukeout.github.io/) -- practice using CSS selectors, you should be comfortable using everything up through the all (*) selector
- Learn Layout (http://learnlayout.com/) -- a really friendly and important guide to essential CSS layout properties
- Micro clearfix (http://nicolasgallagher.com/micro-clearfix-hack/) -- essential to know how to clearfix when you float things
- Codrops CSS reference (http://tympanus.net/codrops/css_reference/) -- super underrated but really really good. For some reason this never shows up in Google results but if you need to look up a CSS property it's worth going here and Ctrl-F'ing on the page itself
- Advanced CSS selectors: levels 11+ (http://flukeout.github.io/) -- selectors after the all (*) selector are pretty advanced
- Flexbox guide (https://css-tricks.com/snippets/css/a-guide-to-flexbox/) -- a guide to CSS3 flex box, which isn't universally supported by browsers yet but still important to know for advanced CSS
- CSSTricks snippets (https://css-tricks.com/snippets/css/) -- a cool place to learn neat tricks
- MDN CSS reference (https://developer.mozilla.org/en-US/docs/Web/CSS/Reference) -- significantly more technical/dense than Codrops, but good when you feel pretty comfortable and just need to quickly double-check something
- jQuery fundamentals (http://jqfundamentals.com/) -- covers basic JS too
- Eloquent JavaScript (http://eloquentjavascript.net/) -- an ebook about JS. I would recommend reading the first 4 chapters so you know basic syntax, types, arrays, etc., and chapters about objects, events, and the Document Object Model (DOM)
- Submitting AJAX forms with jQuery (https://scotch.io/tutorials/submitting-ajax-forms-with-jquery)
- Event delegation (https://davidwalsh.name/event-delegate) -- event delegation is a technique for "listening" for events (like key presses and clicks) on a container, instead of listening at every single element inside the container
- You might not need jQuery (http://youmightnotneedjquery.com/) -- great resource for learning how to do things with "vanilla JS" instead of jQuery, a popular JS library that simplifies things like selecting elements and attaching event listeners to parts of the webpage
- JavaScript algorithms (https://mgechev.github.io/javascript-algorithms/index.html) -- essential for algorithms interviews in JS
- Visual explanations of prototypal inheritance and JS objects (https://howtonode.org/object-graphs, https://howtonode.org/object-graphs-2, https://howtonode.org/object-graphs-3) -- a nice visual introduction to some more advanced JS concepts like scope/closures and prototypal inheritance
- JavaScript Garden (http://bonsaiden.github.io/JavaScript-Garden/) -- documenting the weirder parts of JS
- You Don't Know JS (https://github.com/getify/You-Dont-Know-JS) -- ebook about advanced JS concepts
- Intro to regular expressions (http://regexone.com/) -- this was the site I used to learn regular expressions (regex) for the first time
- Regex101 (https://regex101.com/) -- a really helpful tool for testing regular expressions
- Regex golf (http://regex.alf.nu/) -- game for practicing regex
- React.js for people who know just enough jQuery to get by (http://reactfordesigners.com/labs/reactjs-introduction-for-people-who-know-just-enough-jquery-to-get-by/) -- when you feel comfortable with jQuery and want to learn React, which is IMO the biggest and most important modern web framework, I super recommend this tutorial -- it's really clear and amazing