Essential Documentation
There’s plenty of bad documentation on the web. This page will point you only to the most reliable sources.
Mozilla Developer Network
The MDN documentation is practically an industry standard, and most pages offer links to relevant specifications.
HTML Dog
An accessible reference that’s useful for beginners look to clarify points of confusion. For
example, their entry for <section> opens
with a very succinct description.
Can I Use…
Can I Use… is a well respected and frequently updated reference for the state of stadards support in different browser. The site takes a search term and presents a table of browser support, such as for the rem unit.
Remember that even though the latest browsers all support a unit like rem, there are still older browsers in wide circulation. So look at feature detection scripts for rem units, like Modernizr’s, or well documented fallbacks, like specifying a pixel value for older IE—while understanding that that fallback will prevent text-zooming for IE users.
CSS Tricks
Although I’m always suspicious of sites with words like tricks in their name, CSS Tricks is an indispensable resource. For example, their page on CSS Flexbox is an important visual reference.
Do note that some of the HTML in CSS Tricks’s examples can be suspect and outdated, so bring your own semantic HTML to bear on whatever CSS advice they offer.