There is no shortage of places to reference material to help when developing web apps, including a gazillion tutorials and blog posts (like mine) of various quality, and more importantly – based on the state of play at the time they were written, which could be any time in the last twenty years. I keep bumping up against this – great, clear explanations addressing whatever I was googling, but which turn out to use out of date bits.
I’m loving a couple of good sources:
w3schools.com
The search is good, but the navigation tools are excellent. There are sections for HTML, CSS, JavaScript and so on, but I usually end up in the How To section where there are really clear, minimal examples of how to accomplish some common task with the HTML, CSS & JavaScript for that task.
MDM Web Docs
I’m not clear on what the relationship between MDM and Mozilla is, but this site is at least hosted by them. This is what I’ve been using as a reference for specific things – for example if I want to know the exact methods of an Event, this is the place to go to get the full list. As a bonus, they are a bit opinionated. For example, the page for KeyboardEvent has a handy warning about the edge case of psychopaths using non-standard keyboard layouts and therefore making Keyboard.code complicated.
Good place for details, especially when you know the property you’re interested in and want to see the values and any associated properties.
Can I Use
Once you’ve found the language feature, tag or property you want to use, you need to know if it will work. This is where to find out.
dev.endevour 
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