The MDN HTML page starts with a definiton of what HTML is with links for further study. There are tutorials provided ranging from beginner to advanced. There is also an extensive reference section also. The reference section would be most helpful in my opinion. Any tag you can think of and all the ones you can't are listed here. Or even better if you go to the attribute reference page the elements are listed along with their corresponding attributes. Then click on the attribute and you can actually see them function!
The MDN CSS page also starts with a brief definition of what CSS is with links for further study. Just like the HTML page there are tutorials and reference materials available also. Again the reference section is amazing! Want to learn about the box model it's there. Or perhaps you need to brush up on the grid layout, it's there also. If you need information about how to cook a zucchini however better use google.
In summary MDN is a one stop shop for what you need to know. Of all the resources I've found, this one is the best. It's presented in a no frills way that just facilitates learning. Plus since MDN is opensource and collaborative you can be fairly sure that the information you find there won't have any hidden bias or agenda behind it. A trusted resource for programming help is priceless, and MDN is free to boot!