The main page for HTML (MDN HTML Page) on the MDN site explains what HTML stands for and what it does. It also contains a menu with links to a number of additional resources. There is a link to a basic section with modules to get started using HTML and how to incorporate different functions such as multimedia, tables and forms. There is a link to a tutorial section which gives practice examples. There is a link to a reference area which gives the correct code and explanations for the various elements and attributes. I have personally made great use of this reference feature in my previous class and found it an invaluable tool.
The main page under the CSS heading (MDN CSS Page) is an introduction to CSS and its use. It also has a menu leading to numerous topics on the subject. There is a link to a section to learn the basics of layout and styling. There is a reference area explaining the various components of the CSS rules such as selectors, properties, pseudo-classes and pseudo-elements. There is also a section of guides to using the box model, flexbox, grids and media queries and many other CSS tools.
I was introduced to the MDN website in my previous class and found it to be one of my go-to tools. It is important to have access to trustworthy information as using correct semantics is so important in providing a website that follows all the current and acceptable conventions. This insures that the site is accessible to all and will provide a enjoyable experience for visitors to the site.