latex2e-reference – A reference for LaTeX in HTML
The HTML hyper-document provides a thorough (though slightly dated) discussion of LaTeX 2e.
Maybe you are interested in the following packages as well.
- texbytopic: Freed version of the book TeX by Topic
- docsurvey: A survey of LaTeX documentation
- wtref: Extend LaTeX’s cross-reference system
- fancyref: A LaTeX package for fancy cross-referencing