The information on this page offers a thorough, yet basic overview of fundamental HTML concepts. The side bar also contains references for elements, attributes, and input types. There are also many guides on how to use elements and various other complex topics. The information in the tutorials ranges from beginner to advanced topics.
This page covers CSS, a stylesheet language used to design the appearance of a document written in HTML or XML. It is a core language of the web and is standardized across web browsers according to W3C specifications. MDN contains a vast amount of information on CSS, found in the side bar of this page. Topics range from beginner to advanced level CSS, and covers the fundamentals of several CSS properties and values.
Each page contains a plethora of useful information. There is a wide range of topics that cover HTML/CSS fundamentals from both beginner to advanced levels between the two pages. I would use this as a resource to learn how to better code/design my own works, or for troubleshooting problems in my own code that I'm struggling with. A trusted resource is important when looking for programming help because you want to write good code that works, validates, and follows the standardized W3C specifications