MDN HTML Page

The MDN HTML Page gives general description of what HTML is, how it operates, and various other things. It explains what Hypertext means (the connecting of webpages across the internet), what Markup means, and a vast number of resources for learning and using HTML, ranging from important, to simple, to advanced, etc.

MDN CSS

The MDN CSS Page gives a general explanation of what CSS is, discussing it's connection to HTML and XML, explaining it's use with rendering elements on screen or media, and also gives a fairly detailed history on the usage of CSS and it's versions before going versionless. They also give links to developed editions of CSS, as well as a large number of resources and tutorials like with the HTML page.

Summary

In summary, MDN is a great hub for all sorts of Web related coding resources, giving you both basic and advanced tutorials for HTML, CSS, and much more.