W3Schools is a web developers information website, with tutorials and references on web development languages such as HTML, CSS, JavaScript, PHP, SQL, and JQuery, covering most aspects of web programming.
W3Schools presents thousands of code examples. By using the online editor provided, readers can edit the examples and execute the code experimentally.
as title suggest, one of best reference manual for frontend development (Javascript, HTML, CSS)
For javascript, Mozilla Developer Network is the most authoritative source though
https://developer.mozilla.org/en-US/docs/Web/JavaScript
great web developer tutorial
but looks like hae compability issues with firefox and microsoft edge
I have learned a lot from this website. Also, its css-framework is good.