Mozilla Developer Network (now MDN Web Docs) is great, probably the best Web development reference site from them all. And therefor even Microsoft defaults to us now in Visual Studio Code.

Snippet from they Release Notes for 1.38.0:

Languages

MDN Reference for HTML and CSS

VS Code now displays a URL pointing to the relevant MDN Reference in completion and hover of HTML & CSS entities:

We thank the MDN documentation team for their effort in curating mdn-data / mdn-browser-compat-data and making MDN resources easily accessible by VS Code.