MDN HTML

The MDN HTML reference page contains tutorials as well as references about every HTML element and attribute available. Tutorials include basics HTML and advanced topics such as preloading and CORS. Each element page contains a detailed description, as well as an in-page code editor that you can see the results of real time. It also provides a breakdown of the element's attributes, as well as browser compatibility.

MDN CSS

The MDN CSS reference page also contains tutorials and references for every selector, property and concept relating to CSS. Similar to the HTML section, there is an in-page code editor that allows you to see the impact of changes you've made in real time. MDN also provides a list of compatibility for each property.

Summary

Having a trusted resource such as MDN is important because there are many resources for coding online, but having a well known resource allows for a single trustworthy source, and it means the information you are viewing is more likely to be up to date with industry standard practices and emerging technologies.