MDN HTML
MDN HTMLThe MDN HTML page includes details on each HTML element and attribute, as well as a comprehensive guide to basic HTML concepts.
MDN CSS
MDN CSSThe MDN CSS page includes references for each CSS selector, property and value. It also includes comprehenisve guides to basic and complex CSS concepts. It has in browser support for testing each property and value.
Summary
A trusted resource is important when seeking help because anyone can put anything on the internet, but trusted sources are subject to a large amount of scrutiny, and have a professional interest in maintaining accurate information.